Laser Diode Drivers

In many cases, a diode driver simply needs to supply a constant operation current, resulting in continuous-wave operation of the laser with approximately constant optical output power. Due to the highly nonlinear voltage–current characteristics of laser diodes with a low differential impedance (high dI/dV) (see Figure 1), which also significantly depend on the junction temperature, it is usually not sufficient to apply a certain constant voltage; instead, the electrical current needs to be stabilized by automatically adjusting the applied voltage. This constant current mode is the function of a so-called current source. For increasing device temperature, the curve in Figure 1 is shifted to the right; one then requires a lower voltage for the same current. If one would instead keep the voltage constant, the current could rise substantially, which would further increase the junction temperature. This might even result in a run-away situation, where the laser diode is destroyed, if the current is not limited.

Note that the drive current and not the voltage determines the rate with which electrical carriers are injected into the junction of the laser diode. Therefore, the optical output power is strongly linked to the drive current and less directly to the drive voltage.

For preventing damage of a laser diode, it is important to avoid any excessive drive currents; even short current spikes could destroy a laser diode, e.g. in the form of catastrophic optical damage due to excessive optical intensity as the diode's output facet. Particularly if a laser diodes driver is not made for diodes with a specific maximum drive current, it should have a separate control where the maximum drive current can be adjusted, and the limit set there should be respected by any other controls, e.g. the one which is regularly used to adjust the drive current. (When the current limit is reached, the current may either be clamped to the limit value or switched off until the user reactivates the device e.g. by pushing a button.) In that way, one can greatly reduce the risk that a user accidentally draws the power knob too far; special care has to be applied only when the current limit is set.

In some cases, laser diodes are intentionally overdriven, i.e., operated with a drive current above the recommended maximum. In that way, one may achieve an increased output power, but at the cost of a reduced laser lifetime, and possibly even with the risk of instant damage.

Obviously, a laser diode driver should be relatively immune against external influences such as voltage spikes on the electrical grid or current transients resulting from faulty electrical contacts.

Laser diodes and therefore also laser diodes drivers are available in a very wide range of powers. Some low-power diodes require drive currents of only e.g. 20 mA, whereas high-power diode bars may be operated with drive currents of dozens of amperes. In case of high-power diode drivers, it is of interest to have a switching power supply and related control electronics with high power conversion efficiency – not only in order to save electricity, but also to limit the amount of waste heat which normally needs to be dissipated with additional means such as a ventilator or a water cooling system.

If the system contains multiple laser diodes, one in principle use a separate driver for each one. However, as long as independent power control of these diodes is not required, it is simpler, more convenient and more economical to operate multiple diodes with a single diode driver. Usually, the laser diodes are connected in series, since this guarantees that they are all operated with the same drive current; if they would be connected in parallel, the hottest diode may consume the largest part of the current and thus become even hotter. Also, parallel connections could lead to excessive drive currents in the electrical cables and connectors, also to a reduced efficiency of the driver, whereas higher voltages are often not a problem. For cases with a very large number of laser diodes, one may group the diodes into packages where there is a serial connection of the diodes within each package, and the different packages are driven by separate output stages of the driver (with separate current stabilization). Obviously, the via ring of the used laser diodes (e.g. common cathode or common anode) must fit to the connections of the used laser diode driver.

It is often advisable not to use a laser diode driver which is designed for a much higher drive current than required. Even if a proper current limit can be set, the accuracy of the set operation current may otherwise be worse, the current noise may be higher, and the transient protection may be not sufficiently sensitive for a low-power diode.

Beyond the mentioned basic functionality, laser diodes drivers can offer a number of additional functions:

Interlock Systems

Constant Power Mode

Electrical Monitoring Outputs

Protective Features

Temperature Control and Monitoring

Low-noise Operation

Slow Start Feature and Turn-on Delay

Wavelength Tuning

Quasi-continuous Wave Operation

Short and Ultrashort Pulse Generation

Current Modulation

Computer Control

Some laser diodes drivers are made as instruments specifically for use in laboratories. Here, the user usually has direct access to the device and its controls, which usually include various knobs, buttons and switches, e.g. for adjusting the diode current or temporarily switching it off. Laboratory diode drivers usually have a front panel with a digital display for the diode current, possibly also for other quantities like the applied voltage or the diode temperature, if the latter can be measured with a built-in temperature sensor. Such devices are often not built for a specific type of laser diode, but with increased flexibility for using different diodes. This implies that different maximum voltages and currents can be applied.

Some devices have a standardized housing for mounting in a rack, where they can be combined with other electronic devices.

Finally, there are driver modules for integration into other devices (e.g. mounted on a chassis heat sink) and sold as OEM packages, if not produced by the manufacturer of the system. They often have only electronic interfaces for interaction with other parts of the system electronics, and not directly with the user of the laser device. They are often used in larger quantities and available at lower prices.

Laser diodes are generally not suitable for “hot plugging”: they should be connected or disconnected only while the diode driver is switched off, and proper precautions have to be taken to avoid damage by electrostatic discharge (ESD). For example, one may shorten the pins at the diode before disconnecting the wires from the driver.

Before using a diode driver within some larger system, one should check whether problems could arise due to improper grounding. Some diode drivers have one output pin connected to the case and to earth ground, and may then interfere with additional grounding on other devices. Even if the same pin is connected to earth ground at different locations, problems may result due to a “ground loop” in which magnetic fields can induce disturbing currents.

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