Laser diodes are exquisitely sensitive to electrical disturbances. Laser-diode controllers, unlike general-purpose power supplies, provide constant current to laser diodes as well as solutions to system-design tasks.
The laser diode has become one of the staples of the photonics industry. Compared to conventional sources of coherent light, they possess many inherent advantages: small size, efficient conversion of electrical to optical power, an enormous range of wavelengths and power levels, and low cost per photon. But as many users can attest, the proper electrical setup and control of laser diodes presents some unique challenges. Indeed, as one employee of a laser-diode manufacturer said, destruction of devices in the hands of inexperienced users, either by improper handling or by the use of inappropriate electronics, is a reliable source of repeat business.
The basic electrical characteristics of a laser diode are not difficult to describe. The term "laser diode" is apt, since a laser diode functions as a circuit component in a manner analogous to the more common rectifier diode. Current flows relatively freely through the diode in one direction (from anode to cathode), but is blocked in the other direction. This directionality is what makes rectifier diodes useful, but is an incidental characteristic of laser diodes. Their usefulness, obviously, stems from their ability to emit laser light in response to a current.
In two important respects, laser diodes are more delicate than their electrical cousins. An excessive current flow in the forward direction can cause catastrophic optical damage, the result of too much laser light striking the output facet. A slight backward voltage, or "reverse bias," will also have disastrous consequences. Whereas the reverse breakdown voltage of conventional diodes is typically hundreds of volts, the same parameter for laser diodes is only a few volts. Therefore, proper care of a laser diode requires careful avoidance of both overcurrent and back-bias conditions.
The term "power supply" will be used here to describe a general-purpose device that provides bulk power to an electrical circuit. The term "laser-diode controller" describes a specialized device that is used for the specific purpose of providing current to a laser diode. The differences between power supplies and laser-diode controllers are important; the desirability of using the latter for any laser-diode operation must be emphasized.
The purpose of a power supply is straightforward—to function as a regulated source of DC power for some kind of electrical circuit. A typical power supply is operated by two knobs (or their digital equivalent): one that sets a voltage limit and one that sets a current limit. At turn-on, the electrical output rises until one of the two limits is reached. The power supply's basic job is to maintain its output at one of its limits. This is desirable for most kinds of electrical circuits, but for laser diodes it has numerous shortcomings.
Like other types of diodes, laser diodes exhibit a sharp initial rise in voltage as current is applied. Once this threshold current is exceeded, the voltage rises very gradually thereafter when the current is further increased. For this reason, driving a laser diode with a standard power supply is very risky. At power on, the voltage servo of a standard power supply detects an output value that is below target. It increases the voltage as rapidly as possible. The current is still far below its limit, so the power supply's current limiting circuit is not active. As the laser diode's threshold voltage is reached, the current rises abruptly. The current-limiting circuit now tries to stop further increases, but cannot act instantaneously. A momentary excess of current will occur. Repeatedly subjecting a laser diode to this excess current will shorten its lifetime significantly.
The ideal laser-diode controller is designed from the ground up as a constant-current source, and does not have a constant-voltage mode. There is only one knob (or its digital equivalent), and only one current servo loop. This allows the controller to be carefully designed to virtually eliminate the possibility of current overshoots. A well-designed controller also has a user-settable current limit that prevents the operator from inadvertently requesting a current that would damage the diode.
While the primary purpose of a laser-diode controller may be to supply current, its priority must be to protect the laser diode. While the power supply's mission is to keep the current flowing if at all possible, the controller's mission is to shut off its output quickly when an anomaly is detected.
Laser-diode controllers monitor the voltage across the load to ensure that it is within the proper range at all times. An overvoltage condition indicates that the laser diode may be disconnected. The controller must shut off its output to prevent the user from reinserting the diode into a live circuit, which would likely be fatal to the diode.
Laser diodes in series
It is increasingly common to operate a group of laser diodes electrically in series, powered by a single controller. Although this arrangement is often a simple and cost-effective way to obtain high optical power, there is an electrical scenario in which a failure of a single device places an entire string of diodes at risk. If one of the diodes fails and becomes an electrical short, the controller will experience a sudden decrease in its load voltage. To understand why this is a problem, it is necessary to consider some aspects of power-supply design.
A simplified schematic of a constant-current power source is representative of the type found in laser-diode controllers (see Fig. 1). A switch-mode power supply is actively controlled by a current-sensing amplifier, which adjusts the power supply's output to maintain the laser-diode current at the desired set point. This is an inherently advantageous architecture because its efficiency (the percentage of raw input power that reaches the load) is quite high. Both power supplies and laser-diode controllers, especially ones with high output-current capacity, are of the switch-mode type. Such power sources need an output filter capacitor to smooth the inherently noisy power from the switching regulator. This capacitor is the instantaneous source of current through the diode.
FIGURE 1. A simplified schematic shows a laser-diode control circuit that will produce a constant laser-diode current of 0.33 A. This current can be adjusted by changing the voltage Vref.
In the scenario under discussion, when a short occurs in one of the diodes in a series, the voltage across the capacitor suddenly decreases and the capacitor will supply a large instantaneous current. The current-regulation circuit will eventually respond to bring the current back down to its set point, but this is often too slow to save the remaining diodes from destruction.
A closely related and equally deadly scenario appears when the connection between the laser diode and the power source is intermittently lost and restored. This can be caused by a faulty connector or loose connection, and can occur with single diodes as well as multiple diodes in series. When the connection is lost, the current regulation-circuit senses the decrease in current and tries to raise the output voltage in an attempt to compensate. This initially does no harm, but when the intermittent connection is suddenly restored an immediate overcurrent condition exists. The current regulation circuit will recover, but this takes time and the overcurrent damage is virtually instantaneous. An overvoltage protection circuit is not a certain solution, since it is difficult for such a circuit to be sufficiently fast and at the same time immune from noise and nuisance shutdowns. These are challenging problems, virtually impossible to address with conventional power supplies. Laser-diode controllers are now becoming available that offer effective solutions.
Laser-diode controllers also provide answers to other system-design tasks. Optical safety concerns dictate the presence of an interlock circuit, which may be needed for regulatory compliance. An initial start-up delay—a specific time interval between power on and laser emission—is required to meet the standards of CDRH (the U.S. Food and Drug Administration's Center for Devices and Radiological Health), which also mandates a system-key switch that prevents laser emission when in the off position.
Many applications, particularly at lower optical power levels, need some form of constant-power servo that regulates the light output, rather than the current, of a laser diode. This requires an additional servo mechanism that senses the laser power and adjusts the current accordingly. This must be done in accordance with the prime requirement to avoid over-current conditions, which might result from a simple blockage of the beam to the photodetector or a failure of the photodetector circuit.
Some laser-diode bars and stacks are affected by abrupt changes in current. The concern here is not immediate destruction of the device, but rather a progressive degradation caused by repeated thermal shocks. Laser-diode controllers for such devices must incorporate a slow current ramp, yet allow for occasional rapid safety-related shut offs. To make matters more complicated, the maximum current change rate can vary from one laser-diode type to another. Some controllers accommodate this situation by offering a programmable current ramp rate, allowing the same controller to be used with a variety of laser diodes.
Because of the susceptibility of laser diodes to destruction by back bias, they are extremely sensitive to electrostatic discharge (ESD). Laser-diode controllers reduce the magnitude of this problem by incorporating ESD protection circuitry, but users must still observe ESD prevention practices when handling laser diodes. Back-bias issues also become important when pulsing laser diodes on and off. Transmission-line effects in the cables connecting the diodes with the power source can result in voltage surges of damaging magnitude. Laser-diode controllers offer some protection against this, although care by the operator and the proper cable length is still required.
Temperature control and monitoring is often necessary in laser-diode applications because laser-diode output power and wavelength are both temperature dependent (see Fig. 2). Thermoelectric coolers (TECs) are commonly used to regulate the temperature of the diodes and possibly other critical optical components. Thus TEC control and temperature-stabilization algorithms are frequently required on laser-diode systems. In situations that do not need active control of temperature, it is still desirable to monitor the temperature of the laser diode to protect against over- or under-temperature conditions. Attempting to operate a laser diode when it is over-temperature can cause catastrophic destruction, and at low temperatures there is risk of damage from laser power striking condensation on optical surfaces.
FIGURE 2. A laser-diode controller performs many sensing, monitoring, and control functions.
Every laser, even in the most basic research applications, is always part of a larger system. While the safety and protection of the laser diode is an important focus, the list of features in a laser-diode-control task can become quite extensive when one considers the interface between the laser and the application as a whole. Taken all together, meeting all the requirements for operating a laser diode safely and correctly presents a challenging task.