LASER MODULATION: Fast drivers offer fine control of green lasers

Controlling green lasers with the kind of finesse with which red lasers are routinely run is no mean feat.

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Controlling green lasers with the kind of finesse with which red lasers are routinely run is no mean feat. While semiconductor experts aim for reliable green diodes in a more “direct-drive” approach, the most common way to generate green light remains the frequency-doubled neodymium-based solid state device.

Modulating the output of such multi-stage lasers with precision is made difficult by the simultaneous need to optimize several parameters, such as diode current and crystal temperature, based solely on an output power measured by a photodiode. What’s more, the high thermal loads in such systems mean it takes minutes or more to reach a constant set power level.

Now, RGB Uniwave (Brecon, Wales, UK) is rolling out its solution for truly fine control of green lasers. Developed using lessons learned from driving red lasers, the company is producing laser driver systems that can control green output linearly with repetition rates up to megahertz levels with an arbitrary waveform. The drivers do away with the typical power-hungry thermoelectric (TEC) crystal cooling, leading to efficient control of output within 1%. The systems reach their specified power level in just microseconds.

A total rethink

The drivers produced by RGB Uniwave have a long history, starting with a company called Vector Technology, founded by Ken Owen. Joined by electronics engineer Keith Snook, the company provided a number of pioneering laser solutions, in particular for laser communications.

The need for the control required by such communications system taught the pair a lot about laser modulation, and they have gone on to produce laser drivers for Global Laser, the company that grew from Vector. But having formed RGB Uniwave, they are set to tackle the new challenges presented by the “three-part” nature of typical green lasers (diodes, laser crystal, and doubling crystal). “The three-part system is the hardest thing you can do,” says Owen. “There are two interfaces that are crucial to stability. If you don’t want problems with the crystals, then you can control everything at a specified temperature—but then you can’t do other things like modulating, at least not electronically. You need to go through some form of optical chopper system at the output. What we’re doing is direct modulation.”

That modulation solution is perfectly linear in input voltage, so that any output power waveform can be generated simply. It bumps up the repetition rates that can be achieved, from an industry standard of around a kilohertz to megahertz levels. The system’s performance is constant across a temperature range from 5°C to 40°C, because it can quickly adjust for the sporadic oscillations typical in three-part lasers. And because there is no TEC cooling, there’s lots less current in and heat to be dissipated. The system also incorporates a number of safety features to prevent any overdriving of the diodes.

Owen is guarded about the secret to the drivers, but considers the company’s approach a “total rethink” of the way that automatic power control is accomplished. “The techniques we use are totally different,” he says. “Most people when they build these systems treat them as voltage devices. We treat it as it is, a current device—we actually use the devices in a different way.”

Simpler solutions

Owen says the first job, now that the technology is proven, is to integrate the driver electronics onto a single chip. He sees a bright future for OEM driver systems and complete turnkey systems in the future. But Owen says his company won’t be rushing to put a one-box solution out on shelves. “We consider ourselves developers of enabling technologies,” Owen says. “There are areas where we know the technology will be useful, and we’re willing to work with people to develop solutions for them. There are small companies out there that won’t otherwise have the wherewithal to compete.”

Such fast modulation is promising for laser communications in the green, and the team has installed a system to send music between the Anglican and Catholic cathedrals in Liverpool as part of the city’s European City of Culture celebrations (see figure). Moreover, Owen says, because the systems work over wide temperature ranges at low current consumption, they would be ideal for battery use, use in hot climates, and in machine-vision applications in the meat and fish processing industries.

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A 50 mW analog modulated green laser links the Anglican and Catholic cathedrals in Liverpool, England, as part of the European City of Culture 2008. The laser will carry audio between the two buildings as well as being a visible link between the two faiths.
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D. Jason Palmer

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