Making Lasers More Colorful

Scientists in Sweden have demonstrated a compact device that, when combined with a laser, can generate a wide range of wavelengths that standard lasers can’t achieve on their own. The new laser add-on could lead to compact, inexpensive detectors for explosives or biological and chemical weapons.

The device, reported in the current issue of Nature Photonics, is a mirrorless optical parametric oscillator (OPO). OPOs, which have been in use for decades, take light from a laser source and convert it into wavelengths that the laser can’t generate itself, allowing scientists to pick the best color of light for a particular application. Many organic molecules, for instance, are easiest to identify at certain infrared wavelengths, which is useful when authorities are looking for biological contaminants or explosives.

Traditional OPOs are a combination of crystals, usually made of lithium niobate, and mirrors. But the mirrors pose a problem: they are hard to align, as well as bulky–a typical OPO is nearly as long as the average dining-room table.

Forty years ago, scientists designed an OPO without the mirrors; however, it required new materials or fabrication techniques that weren’t available. Now Carlota Canalias and Valdas Pasiskevicius, two researchers in the Applied Physics Department at the Royal Institute of Technology, in Stockholm, have succeeded in making a crystal for a mirrorless OPO.

In the standard OPO setup, the crystal splits the original light beam into two lower-energy beams with different wavelengths. The mirrors bounce the beams back and forth through the crystal to amplify them until they combine into one stronger beam of light. If the different beams are out of sync–with the peaks and valleys of the light waves not lining up from one beam to the next–then the beams interfere with one another and produce only weak output. To keep the beams in sync with one another, the crystal is engineered into alternating sections with opposite electrical properties. As the beams pass through each section, they are bent slightly in such a way that they stay aligned.

The key to the new OPO is the crystal, made of a combination of potassium and titanium called potassium-titanyl phosphate, and its design, which amplifies the laser light itself, eliminating the need for mirrors. In order to create the electronically different regions, Canalias says, the researchers employed photolithography, the process used to make transistors on computer chips. Using this method, she explains, they laid down a series of aluminum wires and an insulator, then ran an electrical current through the wires. The electrical field generated by the current changed the crystalline structure of the portions of the crystal underneath the wires.

Without the mirror, Canalias says, “you can have a very compact and nice setup, and you don’t need to worry much about it.”

The OPO provides fine tuning between different wavelengths of infrared light–differences of 5 to 10 nanometers should be easy to achieve–making it ideal to use in spectroscopy to identify different substances, she says. “Whatever wavelength you need, you can generate.” The device maintains the same wavelength even if it heats up with use–something that traditional OPOs don’t do so easily.

Franco Wong, a senior research scientist at MIT’s Research Laboratory of Electronics, says that a mirrorless OPO would be very useful because it wouldn’t need complicated adjustments. It might make for a portable spectrometer that could easily be carried onto public transportation to check for chemicals found in explosives. “It’s almost self-aligning,” he says. “You set things up, and it would just operate.”

Yujie Ding, a professor of computer and electrical engineering at Lehigh University, has been working on mirrorless OPOs for years, and he says that Canalias and Pasiskevicius have confirmed experimentally a structure that he and his colleagues described 11 years ago. While he’s happy to have his predictions validated, Ding is skeptical that the experimental results can translate into something practical. Right now, the researchers’ device requires a large, high-power laser, he says. In order for the new OPO to work with smaller, lower-power lasers, the banded crystal would need to be more than 10 times longer than its current 0.4-millimeter length. But due to the properties of the crystal, it’s difficult to make the bands uniform in a bigger piece of crystal, Ding says.

Canalias says that she has plans to try to make the crystal longer and at the same time decrease the width of the bands by at least half to gain even greater control over the quality of the output beam. She suspects that within the next 10 years this prototype could find its way into a practical commercial device.