Design and Stability Analysis of a Digital Automatic Power Control Based on a PI Controller for Laser Drivers  
[PERSON]  
1 Electrical Engineering Department, Faculty of Physical and Mathematical Sciences, University of Chile, Santiago 8370448, Chile; [EMAIL_ADDRESS] (J.P.); [EMAIL_ADDRESS] (A.B. ); [EMAIL_ADDRESS] (J.R.)2 Space and Planetary Exploration Laboratory (SPEL), Faculty of Physical and Mathematical Sciences, University of Chile, Santiago 8370448, Chile  
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[PERSON]  
1 Electrical Engineering Department, Faculty of Physical and Mathematical Sciences, University of Chile, Santiago 8370448, Chile; [EMAIL_ADDRESS] (J.P.); [EMAIL_ADDRESS] (A.B. ); [EMAIL_ADDRESS] (J.R.)2 Space and Planetary Exploration Laboratory (SPEL), Faculty of Physical and Mathematical Sciences, University of Chile, Santiago 8370448, Chile  
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[PERSON]  
1 Electrical Engineering Department, Faculty of Physical and Mathematical Sciences, University of Chile, Santiago 8370448, Chile; [EMAIL_ADDRESS] (J.P.); [EMAIL_ADDRESS] (A.B. ); [EMAIL_ADDRESS] (J.R.)2 Space and Planetary Exploration Laboratory (SPEL), Faculty of Physical and Mathematical Sciences, University of Chile, Santiago 8370448, Chile  
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[PERSON]  
3 Crostian.Pavez  
the optical power [5]. Moreover, low-frequency instabilities in the emitted power can cause a reduction in detection thresholds, generating an increase in the bit error rate or even a total disruption of the communication. Laser diodes exhibit an ultra-fast dynamic response, on the order of nanoseconds, which allows them to attain their steady state within a short period of time--typically in the range of hundreds of nanoseconds. Consequently, the development of a control system that manages the dynamics of the laser can be expensive and technically difficult. Therefore, the majority of the controllers assume that the steady state is instantaneously reached after a control parameter is changed. The optical power stabilization methods can be classified as internal and external control systems. The internal control modifies the internal parameters of the laser operation, such as the forward current or supplies current, temperature, and cavity, among others [11; 12; 13]. On the other hand, the external control systems require external modulators, such as opto-acoustic or electro-optics modulators in the laser outlet, to modify or stabilize the optical parameters of the laser. Although the external control systems are highly precise, they require large volume systems to stabilize the optical power, which can be impractical for some mobile and space applications. Internal control systems employ various approaches. The more common assumption in this type of solution is precisely measuring and controlling the temperature [14] and/or supply current [15], thereby, resulting in a highly stable optical power. The advantage of these types of solutions is that the system can be implemented by using microcontrollers and commercial current drivers, along with thermal sensors, to operate and safeguard the laser [16; 17]. Although these drivers can achieve high precision in emitted optical power, they are unable to compensate for changes in optical power caused by aging or internal laser issues because they do not measure the actual laser emitted power. Automatic power control (APC) is a feedback laser driver that avoids the previously mentioned issue. It acts over the internal parameters of the laser, typically the current supply and forward current, using (as a feedback signal) the actual optical power emitted by the laser. The APC can be implemented either with analog or digital circuits. The analog APC circuits require defining a bias current as the supply current needed to operate the laser at the set-up optical power at a stable operating temperature. This current is analogically modified by the feedback signal to compensate for effects produced by temperature changes and the aging of the laser, among others [18]. The feedback signal is obtained with a probing photodiode, typically included in the laser packaging.