Laser Diode Drivers
Laser diodes drivers are electronic devices which are used to supply one or
several laser diodes with the required electrical drive current. Most of them
obtain electrical power from the public grid, but there are also battery-operated
devices.
Basic Functionality
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 electric 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.
Figure 1: Current vs. applied voltage for an 808-nm laser diode. For a current
of 1.2 A, as needed for the nominal output power of 1 W, the required voltage is
roughly 1.8 V. (For comparison, the photon energy for 808 nm is 1.53 eV.) At
elevated junction temperatures, this curves somewhat shifts to the left.
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.
Different Power Levels
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 the 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.
Additional Functionality
Beyond the mentioned basic functionality, laser diodes drivers can offer a
number of additional functions:
Interlock Systems
Very often, a diode driver has some interlock system, which can switch off the
laser in case that a certain safety condition is detected – for example, and
opened device housing. It can be very useful to have multiple interlock
connections for implementing advanced safety features. Some of them may
treat conditions like insufficient coolant flow on a chiller in order to avoid
hardware defects.
Constant Power Mode
There are devices which can stabilize the optical output power (constant
power mode), based on a signal from the photodetector, which may be
integrated into the laser diode package. (That is particularly often the case
for fiber-coupled laser diodes.) Of course, a certain maximum drive current
must never be exceeded; otherwise, a laser diode could be killed as a result of a
faulty photodetector signal. Often, it is possible to switch between constant
current mode and constant power mode.
Electrical Monitoring Outputs
There may be electrical outputs, e.g. delivering a voltage proportional to the
laser diode current or the monitored optical power, possibly with a calibration
feature.
Protective Features
The applied voltage should be monitored, and if an unusual operation voltage is
detected, the device may switch off the diode in order to prevent damage. If
several laser diodes are operated in series, a sudden drop of the voltage may
indicate the death of one of the diodes, and it may then be wise to investigate
the situation before the other laser diodes are also destroyed. Also, it is useful if
the driver recognizes wrong poling of a diode because it could be destroyed by
an excessive reverse voltage.
For transient protection, the diode's cables should not simply be disconnected
when switched off, but rather electrically connected together (shorted), so that
electrostatic discharges cannot build up a voltage across the pins.
Temperature Control and Monitoring
Some devices have an integrated temperature controller, driving e.g. a Peltier
element based on the signal of some temperature sensor. Even without a
temperature stabilization feature, it can be useful to monitor the junction
temperature for switching off the laser before it gets too hot. Alternatively, one
may monitor the emission wavelengths, which reacts sensitively to temperature
changes, unless an optical wavelength stabilization is used, e.g. based on
optical feedback from a volume Bragg grating.
Also, it is sensible to monitor the internal temperature of the electronic driver
device, since overheating e.g. due to a blocked air or water flow may destroy
the driver and possibly the laser diode(s) in addition.
Low-noise Operation
Some drivers are made for operation with a particularly low current noise. This
can be important, for example, when driving lasers for sensitive optical
measurements. Low-noise operation is mostly offered for low-power devices.
Slow Start Feature and Turn-on Delay
Particularly for high-power laser diodes, it can be useful to limit the rate with
which the current can be ramped up and down (slow start feature) because this
reduces the internal mechanical stress related to temperature changes. In
addition, a turn-on delay is often used as a safety feature; people in the room
are warned about coming laser radiation before it is actually turned on.
Wavelength Tuning
Some diode lasers, in particular external-cavity diode lasers, are suitable
for wavelength tuning in a substantial range. For example, a diffraction
grating on a motorized stage can be used for controlling the emission
wavelength. Some laser diodes drivers contain functionality for tuning the
wavelength e.g. via controlling some stepper motor.
Quasi-continuous Wave Operation
Some drivers are suited for quasi-continuous-wave operation (QCW mode).
This means that they can apply current pulses with an adjustable duration e.g.
between 1 μs and 10 ms, which can be triggered with an external electric
signals or with a built-in clock. The possible peak current may be well above the
current which the driver could deliver continuously, or which the laser diode
could tolerate continuously.
Short and Ultrashort Pulse Generation
They are specialized drivers for producing nanosecond or picosecond pulses,
e.g. by gain switching. Here, it is particularly important to select a suitable
laser diode and to properly adjust the parameters of the applied current pulses.
Current Modulation
In other cases, a laser diode driver allows some other kind of modulation of the
supplied current. This can be done in many different forms. For example, a TTL
input signal may be used to switch the current on or off. In other cases, an
analog input signal is added to the base current set with the controls. Drivers
can differ very much in terms of modulation bandwidth and depth of
modulation.
Current modulation is often available only in current control mode, i.e., not in
combination with output power stabilization. This is because the limited
feedback bandwidth of the stabilization circle it would strongly limit the possible
modulation bandwidth.
Computer Control
A diode driver may be computer-controlled, connected e.g. via a USB, GPIB or a
serial interface like RS-232. It may receive inputs, e.g. concerning the requested
drive current, and deliver outputs, e.g. concerning the achieved optical output
power or the required diode voltage.
Different Types of Diode Drivers
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.
Further Remarks
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.
