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Publication Date: 09/1/2010
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Protecting Laser Diodes from ESD and Power Surges
ESD protection in a tiny package.

Laser diodes are very fast devices that react rapidly to changing voltage and current. While this is a primary benefit in many applications such as signaling, the fast reaction time is also a potential liability, because it makes Laser diodes vulnerable to damage from voltage spikes experienced during electrostatic discharge (ESD) events and power surges. BLU-RAY and low-power red laser diodes are particularly susceptible to voltage spikes, since any increase in applied voltage beyond normal operating parameters will cause both optical and electrical overstress.

Anecdotal evidence suggests that many laser diodes fail long before the end of their life expectancy. ESD and power surges are often to blame. These power surges are not those caused by lightning or power line fluctuations, but rather those generated during power on/off cycles. Voltage spikes are often produced at these times, and their cumulative effect on laser diodes is detrimental.

Protection Needed
In 2008, Pangolin was contracted to design a laser diode driver for an aerospace application, where high reliability and effective ESD protection were vital requirements. When designing the laser diode driver, we started with the ESD protection requirement and built several experiments to select the best ESD protection scheme. We assumed that one of the many common ESD components on the market would be effective at preventing damage to a laser diode. In our testing, we tried devices such as TVS diodes, Schottky diodes, Multi-layer varistors and the like. However, when we tested these devices along with a real ESD gun, we found that none of them were able to protect the laser diode.

In our quest to understand why existing design solutions for ESD and surge protection didn't work with laser diodes, we found that they were originally developed to protect components and circuitry that react more slowly than laser diodes, or that they operate at different voltage ranges. A typical shortfall of these components was that they allowed the normal operating voltage to be exceeded, even if only for a brief time, thus allowing excessive forward-bias. Since none of the common ESD components on the market worked with a laser diode, we determined that a new method of protecting laser diodes was needed — one that would act quickly to prevent the voltage during an ESD event or power surge from increasing beyond the normal operating voltage of the laser, and also one that has the capacity to absorb high energy in the circuit until ESD or power surge subsides.

To address these unique protection requirements, we developed an electrical component that we named "LASORB". The word LASORB is a combination of the words "LASER" and "ESD ABSORBER". The LASORB component is easy for engineers to implement because it is small and can be simply connected directly to the pins of the laser diode.

How it Works
Most other protection methods operate on the principle of a fixed "clamping voltage". Although LASORB does have a clamping voltage for both positive and negative potentials, this is not the primary means by which it provides protection.

Instead, LASORB includes a slew-rate detector, which monitors the voltage across the pins of the laser diode. If a fast change-of-voltage condition is detected, this triggers LASORB to aggressively conduct the voltage (and thus current) away from the laser diode. The conduction is initiated very rapidly — in some cases less than one nanosecond — and the conduction can last from several microseconds, to tens of microseconds.

This change-of-voltage approach is a very good principle to use for laser diodes because, under ordinary circumstances, even during modulation, the voltage across laser diode pins does not change very much. Therefore any change of voltage, especially a fast one, indicates an abnormal event, something that happens only during ESD or other types of electrical faults.

In our testing with red, infrared and BLU-RAY laser diodes, LASORB was found to be 100 percent effective at protecting them from positive-ESD events up to 15kV, when using the human body model associated with ANSI/ESD STM5.1 and IEC 61340-3-1. It was also able to protect all laser diodes from negative-ESD up to 15kV when using the human body model associated with ANSI/ESD STM5.1 and IEC 61340-3-1 as well as IEC 61000-4-2.

Customizable Product
To allow for complete flexibility and customizing parts to individual laser diode requirements, the LASORB device is implemented as a hybrid, containing active silicon and passive components. And as a side-effect of the makeup of this hybrid component, it also offers protection from reverse bias conditions, thus allowing it to offer complete protection of the laser diode from a variety of electrical faults.

We designed a series of LASORB parts so that engineers are able to select the most appropriate slew rate, surge conduction time and maximum operating voltage of the part, to provide the best match for the laser diode they want to protect.

Our most recently-developed part is one that is specialized for the newer 445 nanometer blue laser diodes, which are becoming popular for display applications because of their higher visibility and brighter output. In addition to their use in laser projectors, these laser diodes are even being used in place of the classic arc-lamp in data projectors. Casio's new "Green Slim Projector" is an example of one product using 24 of these new 445 nanometer laser diodes.

Other Benefits
In addition to laser diodes, there are other types of opto-electronic devices that have similar ESD and voltage-spike vulnerabilities. These include photodiodes, LEDs, superluminescent diodes, and even Gunn diodes.

Indeed, since we released LASORB, several customers have experimentally determined that it may be used to replace better-known circuit-level ESD protection devices such as Transient Voltage Suppressors (TVS) and Metal Oxide Varistors (MOVs). It is a better choice for circuits that absolutely cannot stand fast changes of voltage in the forward direction, or any voltage in the reverse direction, because it responds actively to prevent fast voltage changes from occurring, while simple clamping-based devices act more like passive limiters. The product is available as a relatively small, through-hole component.

Contact: Pangolin Laser Systems, Inc., 9501 Satellite Boulevard, Suite 109, Orlando, FL 32837 407-299-2088 fax: 407-299-6066 E-mail: Web:  

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