The circuit presented is a low-cost laser-diode driver with current limiting and a lasing monitor for safe operation of the laser. The driver provides temperature stabilization using a built-in thermoelectric-cooler (TEC) controller. This circuit is designed around a QLM5S876 1. 55-m laser-diode module with a built-in monitor photodiode, TEC, and t

hermistor. The laser is driven by the constant-current source Q2 and Q1. The drive current is stabilized against supply changes by Z1, limited by R2, and adjusted by R3 to the desired operating point. When the current is small, the laser doesn`t lase and the monitor photodiode detects no optical power. As a result, comparator U1C goes below its trip point and LED1 turns off. When the current is adjusted and passes the laser threshold, sufficient optical power is generated by the laser, causing photo current to flow through the photodiode and the comparator to drive the lasing monitor indicator LED1 to on. The TEC controller circuit is a feedback system. If the temperature is higher than the set point, the comparator output will be high and will drive the TEC element through R15, Q3, and Q4. The TEC drive current is limited by Z2. When the TEC is driven on, it cools the laser. This increases the voltage of the inverting output until it passes the comparator`s upper trip point and turns off the TEC`s drive current. The temperature set point can be adjusted by R16.

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