Lasers go embedded

October 12, 2015 // By Marko Hepp
Embedded systems and laser applications are based on different technologies. Embedded systems are basically digital and require an analog connection to laser technology. The goal is a compact and efficient solution combining digital embedded systems and analog laser technology.

While the focus is on typical digital embedded platforms such as CPLD, FPGA, MCU, DSP and their derivations, appropriate bridging technology can ease such integration.

Laser diodes have versatile applications such as measurement, material processing and lighting. The combination of high intensity and narrow spectral frequency range of the many different laser diodes is specifically used in numerous applications. Often two laser diodes of different wavelengths are also integrated into one package. With this, differential measurements at defined wavelengths can be implemented. Furthermore, a laser diode can be used for sensors and for processing.

Laser diodes, considered as components, commonly contain in addition to the active, light-emitting laser diode a monitor diode in the optical path that enables an optical power measurement. A variety of packages and wiring variants is available.

Depending on the pin assignment and reference potential, current outputs with common ground or current inputs with common supply are required. The classifications N-, M- and P-type describe the electrical wiring of the laser diode with the integrated monitor diode.

The dynamic behavior of the laser diode and monitor diode has to be considered. The currents of laser diodes range from a few mA to several A and overstrain typical DAC or GPIO port current ranges. Looking at individual laser diodes, the forward voltages range from circa 1 V to 10 V and thus above usual I/O voltages of typical embedded systems. The characteristics of laser diodes are non-linear and require a high accuracy and dynamics at certain operation points.

Compared to other active components, laser diodes have a limited temperature range and radically change their analog characteristics with the temperature. The efficiency of a laser diode is also dependent upon temperature. Often expensive, the laser diodes need to be controlled in a defined way for their protection.

A fast ramping-up power supply can lead to damages of the semiconductor structures in the laser diodes and shorten their service life. The power supply of the laser diodes also influences the stability of the system. Voltage fluctuations and faults, such as ripples or spikes that are caused by the power supply itself, are to be minimized by the laser diode control.

Controlling laser diodes

Regarding typical controls of laser diodes, two groups can be formed: the CW (Continuous Wave)-control and the modulated control. At CW-control a stable and constant operating point is regulated. In case of APC (Automatic Power Control), the current of the laser diode is controlled by optical power. A monitor diode in the optical path measures the optical power and provides feedback to the control circuit, which then adjusts the current of the laser diode. In case of ACC (Automatic Current Control), the current is controlled within the laser diode. A monitor diode is not necessary. A typical example of this fast switching laser driver is shown in fig. 1.

Fig. 1: Fast laser switching in ACC-mode with LVDS-inputs to reach 200MHz at 9A peak

For the modulated control, there are both static and dynamic proportions. Laser diodes are put into laser operation by a constant bias current. An additional modulation signal controls the dynamic proportion of the laser diode control. The modulation ranges from a simple analog, low-frequency modulation to a synchronized, digital pulsed mode in nanosecond range.