Lasers go embedded
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.
From the view of the embedded system, the laser diode control must take many aspects into account. The laser diode control needs to react quickly and with high-precision to changes in regulations. The dynamic characteristics of both the laser diode and the feedback paths also need to be considered.
The required controls and the operation points need to be adjusted for this. The control can be based on the processor as a software-based controller. Alternatively, an integrated, analog controller can be used. An integrated, analog controller processes both the control parameter and feedback directly and analogously. Here, the embedded system has no computing load with the controller and thus can be dimensioned smaller regarding the computing power.
Depending on the system configuration, temperature monitoring can compensate thermal effects of the laser diode characteristics by the control or control parameters respectively. Additionally, aging effects of the laser diode can be compensated for the complete life cycle of the laser system.
Laser systems safety
The safety aspect for laser systems is particularly important. In addition to compliance with laser classes, the detection of failures is important. In case of failure the safe system status must be achieved. In particular critical cases, the system can react autonomously and goes into a safe state.
The laser diode control monitors the operation autonomously and signals the embedded system and its software that there is need for action. Limitations in the supply must be identifiable for the system; if necessary, the control of the laser must be switched off.
A restart after a supply disruption (e.g. with reset parameter set or lost operation point) must be prevented. To start security-related systems, mostly multiple instances are necessary. A combination of hardware-based signals and software-based conditions ensures the starting of the laser diodes.
Digital laser control
The digital control of a laser diode via a laser diode driver includes the digitalization of analog signals and the analog output of digital parameters as well as binary control and feedback signals. Their typical standard interfaces such as SPI or I²C provide advantages in digital communication for parameter and status transfer.
Adjusted to the performance of very short control pulses, I/O standards such as LVDS are used mostly for communication. Embedded systems and laser diodes mostly require different voltages and thus different power supplies. While embedded systems usually use a supply voltage and I/O voltage of 2.5 V or 3.3 V, laser diodes require partly voltages of more than 10 V DC for operation.
Depending on the system configuration, a level adjustment between the embedded system and laser diode control is required additionally. Since the typical currents of the laser diode control range from a few mA to several A, additional power adjustments of the analog current output are required.
A major advantage of all embedded systems is the possibility to use software (e.g. firmware) to extend its functionality. The software of an embedded system can be individually and flexibly extended to a specific application. Safety sequences allow checking the system in use and can react when thresholds are exceeded.
Complex error pattern are identifiable by versatile analysis options. Through “Condition Monitoring”, detailed information on the operating point and system status is provided. The availability of the systems can be monitored and transmitted to higher authorities.
Additionally, functional sequences can be used for the complete “Product Life Cycle”. Already during the production first self-tests can provide early information about the system. Calibrations can be stored non-volatile and updatable secured within the embedded system, and can be digitally exported to external backup. Fig. 2 shows the block diagram for digitally controlled laser diodes by a microcontroller SPI-interface.
Fig. 2: Controlling Laser diodes digitally with a microcontroller SPI-interface.
Special attention must be paid to the total energy output in compact embedded systems. The power dissipation as waste heat of the controller is defined by the voltage drop and the current of the laser diode.
If the power supply of the laser diode is controlled by the overall system, the voltage drop and therefore the power dissipation as waste heat can be minimized to the operating point. The minimization of the system-internal waste heat increases the efficiency of the overall system and widens the usable temperature range.
Hardware-based evaluation
The integration of a laser diode control in embedded systems is simplified by versatile evaluation systems. As a first step, the laser diode control should be particularly simple. At this stage, experience in the application of laser diodes will be acquired.
As the next step, connecting the laser diode control to the existing development platform (see fig. 3) of the embedded system is useful. Here, the own functions and algorithms can already be implemented and tested on the target system. As the final step, the embedded basis and laser diode control are integrated on a separate circuit board/PCB with proprietary software-based functions that have been developed and intensively tested on the development platform.
Fig. 3: PC based development platform for Laser diodes driver.
Integrated Laser-front-end components
Typical components used for a complete laser diode control include an analog-to-digital converter, a digital-to-analog converter, a temperature sensor, an analog regulator, power electronics, a linear regulator, configuration memory, communications interface, etc.
This variety of components, mostly integrated with general interfaces to embedded systems should ideally be integrated into a single, custom component.
The use of integrated components as laser-front-end solutions enables a particularly simple laser diode control and a compact implementation of the overall system. All necessary basic components, base functions, configurations and system states are digitally addressable with standard interfaces for the embedded system.
The typical functions of laser diode controlling are parameterization, voltage monitoring, current monitoring, over-current shutdown, over-temperature shutdown, error handling, safety system, etc. Additional application-specific functions are individually extendable using the software on the embedded system.
By combining application-specific, integrated components and adaptable software, the integration of laser diodes in embedded systems is particularly simple, flexible and efficient. The laser system can be extended with typical embedded features such as digital access, condition monitoring, IOT ability, etc. and is implemented compactly with individual features.
About the author:
Marko Hepp is Application Engineer at iC-Haus GmbH – www.ichaus.de
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