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Ultrashort pulses make 3D devices
12 October 2001

Femtosecond laser oscillator gears up to prototype integrated photonic systems.

X Coupler

Kaoru Minoshima and colleagues from the Massachusetts Institute of Technology (MIT) have created three-dimensional photonic devices with a high-power femtosecond oscillator. They also used optical coherence tomography (OCT) to image the internal structure of the devices (Opt Lett 26 1516).

Instead of amplifying the oscillator output to increase its power, the MIT group increased the length of the cavity design. This gives a more compact systems design when compared with conventional femtosecond lasers.

US researchers from Harvard University and telecoms giant Corning were the first to make glass waveguides and couplers with unamplified femtosecond lasing earlier this year. The laser energies used by the researchers were between 2.8 and 5 nJ.

Minoshima says, however, that femtosecond oscillators with small pulse energies are limited to making simple glass devices. "To make practical and complicated devices we have used pulse energies of up to 100 nJ during fabrication," she said.

"[Because of this] our method is not limited to glass," Minoshima told Optics.Org. "Higher pulse energies mean that it is open to a wide range of materials with higher bandgap energies."

The researchers created planar waveguides by scanning glass plates across the tightly focused incident femtosecond laser. By adjusting the focus of the laser the researchers altered its scanning depth within the plate to create three dimensional structures.

Minoshima and colleagues fabricated a range of waveguides with singlemode and multimode outputs and different refractive indices by altering the femtosecond laser's power and scanning speeds. They hope to combine these structures to make high-density integrated systems, and have already made an X coupler and a stacked, multi-layer waveguide.

To investigate device properties, the team used OCT - a technique usually associated with medical applications. OCT uses ballistic-light signals to detect different depths within a sample.

"Applying OCT in this way is new," explained Minoshima. "It has been a powerful tool in revealing the internal structures of the devices together with refractive index information."

According to the researchers, their biggest challenge is to stabilize the laser. "We need to control the power, beam pointing mode and wavelength," said Minoshima. "By making the laser cavity compact and robust, the stability of the laser oscillator can be improved."

She also plans to make more devices such as resonators, switches, sensors, active waveguides and hybrid systems. The team believes that their technique will one day rapidly prototype these three dimensional photonic devices.

"Realization of this is not far away," concluded Minoshima. "It depends on the laser development, but our studies are already under way."

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