Very Short Pulsed High Diode Currents Generated by a Fast Laser Diod Driver
Above the threshold point the optical power increases basically proportionally with the current through the laser diode. Only with a real current source the diode current and so the optical power can be controlled precisely . Modified standard voltage supplies can never control the diode current in the requested way and have a high risk of current peaks which damage the laser diode. A real current source has the inherent advantage that the output voltage of the driver adjusts itself automatically to the diode voltage.
Therefore, all the following explanations refer to a driver as a real current source.
The biggest physical limitation for a very fast rise time <1µs are the parasitic inductances between driver and laser diode . In parallel cables these inductances are very high and slow down the required fast rise time tremendously.
This shows that the inductance mainly increases with the length of the cables (l) and with the distance between the cables (d).
In order to reduce these inductances the distance between the driver and the diode must be as short as possible. This means that the diode must be mounted directly to the driver. In addition, the cables must be replaced by parallel conducting sheets as close as possible and placed in a way that the inductances compensate each other.
A fast control of the diode current is required for very short rise times of 20ns … 50ns as well as for high frequencies up to 30MHz or for fast alternating set-points. A further requirement is a clear signal shape without overshoot above the set-point which could damage the laser diode. Very fast pulses are used e. g. for medical applications.
The best technical realization for a precise current control is a fast analog circuit without microprocessor in the control loop which has the disadvantages of delays caused by internal cycle time and jitter.
High accuracy control of the diode current is needed for the conversion of the input set-point into a precise diode current as well as for fast compensation of ripples.
Also for this requirement an analog controller with high precision electronic components is preferred because limitations by digital bit resolution of analog / digital converters are avoided. Additionally, an optimized PCB layout prevents from interferences and deviations of fast electronic signals.
High short pulsed diode currents >200A can be generated by special power transistors with fast switching times, low internal resistance, good avalanche behavior and good heat transfer. High currents with short pulses are required e. g. for material treatment.
Capacities at the output side of the driver can cause energy peaks which occur e. g. in case of a defect of a single diode within a diode stack or if the diode is disconnected and connected to the driver again. These energy peaks will damage the laser diode. In order to keep the diode current unchanged capacities at the output side of the driver are not allowed.
Another physical restriction is the skin effect  which is contra-productive especially for high currents. A DC current or a AC current with low frequencies uses the whole wire cross-section. However, due to the skin effect the conduction electrons for currents with high frequencies are forced to the surface of the conductor. Therefore, the full cross-sectional area is not available for the whole diode current anymore, but only the surface areas of the conductor which leads to a higher ohmic resistance.
The requested rectangular signals for pulses are the reason for this effect. According to Fourier analysis these signals consist of an overlay of many high frequency harmonic sine waves.
This additional ohmic resistance depends on the frequency; e.g. the skin deepness of a current with 10MHz is only 21 µm.
Therefore, flat metal sheets (striplines)  with special coating must be used in order to achieve a higher surface cross-section for the current flow.
There are inductances and capacities within the laser diode caused by internal electrical wiring and by assembly of diode components as well as inductances and capacities in the connection between driver and laser diode.
These physical effects influence the control mode for the diode current. Therefore, in combination with the control parameter of the driver there is a high risk that the whole system "fast diode driver – laser diode" will show severe oscillations which could damage the driver and / or the laser diode. The following figures show a clear signal and an output signal with oscillations.
Therefore, the control loop of the fast diode drivers must be adapted and optimized to the inductive and capacitive load situation.
This adaptation and optimization must be done in two steps. In the first step a short mechanical connection according to above described measures must be optimized. Then in a second step the control parameter such as P-component and I-component must be adapted to the control behavior of the whole system "fast diode driver – laser diode". The targets are the fastest possible dynamic behavior with short rise and fall times, steep rising and falling edge as well as avoiding overshoots.
The inductivities and ohmic resistances of long supply cables between the power supply S1 and the driver M1 (see fig. 1) can lead to voltage break downs at the driver input during high energic pulses or fast modulated operation. This must be additionally considered for the rating of the AC / DC power supply S1.
Beside cables with a higher cross-section a buffer capacitor with additional external capacity CEXT provides short-term energy.
Fig. 6 and Fig. 7 show whole set-ups consisting of a laser diode, a fast diode driver and a buffer capacitor completely assembled on a heat sink.
A ripple in the diode current will be transformed into a ripple in the laser light emission. This is an undesirable effect and affects the laser process negatively. Standard drivers cause an alternating output signal on top of the diode current due to internal switched-mode technology. In the opposite, drivers based on a linear current controller technology do not have this effect. The input control signal is exactly converted to the output current without disruptions and alterations.
Unfortunately, linear current controllers have the disadvantage of higher power dissipation. But for short pulses with a normally low duty cycle this power dissipation is manageable.
Formula for short pulses with duty cycle, see fig. 1:
Pv ≤ (U2 – U3) · I2 · duty cycle.
Pv ≤ (12 [V] – 9 [V]) · 200 [A] · 0,05 = 30 [W].
A precise mechanically fine grinded and lapped base plate of the driver in combination with heat transfer paste and a heat sink – ideally made of copper for high requirements – guarantees a sufficient heat transfer from the diode driver to the heat sink which is cooled by air or water.
With the above described measures and with the right electronic design the following further technical requirements can be fulfilled:
• The driver does not need a fan. Fans are noisy and have often a low reliability.
• The dimensions of the driver are small (approx. 100mm · 60mm · 20mm) and the weight is low (approx. 250g), so that the driver can be assembled directly to the diode even in a moving laser head.
• The driver has two overlaying set-point inputs and one BIAS potentiometer so that a CW operation, a pulsed operation, a modulation or any mixed signals with arbitrary curves are possible.
• The fast precise analog electronic circuit also allows a monitoring of the actual diode current in real time.
In the following table the main fast diode driver models are shown.
MESSTEC Power Converter has developed fast laser diode drivers which have integrated best technical solutions and above explained advantages. They can operate in CW, modulated or pulsed mode with any arbitrary curves and can be used in combination with all laser diode manufacturers.
MESSTEC also delivers complete set-ups and electro-mechanical modules with diode driver, laser diode, heat sink and optimized control loop.
List of industries for which MESSTEC drivers are used:
• Additive Manufacturing
• Sapphire Glass Treatment
• Printing Industry
• Marking, Engraving, Labeling
• PCB Manufacturing
• Material Heating
• Surface Processing
• Medical Devices
• Pumping for DPSS Lasers
• Nano Processing
• Very Fast Pulsing
• Plastic Welding
• Safety Technology
• Research Institutes