Current visible light applications for OPAs, say the researchers, have been limited by bulky table-top devices that have a limited field of view due to their large pixel width. Previous OPA research done at the near-infrared wavelength faced fabrication and material challenges in doing similar work at the visible wavelength.
"As the wavelength becomes smaller, the light becomes more sensitive to small changes such as fabrication errors," says Min Chul Shin, a PhD student in the Lipson group and co-lead author of a paper published in Optics Letter . "It also scatters more, resulting in higher loss if fabrication is not perfect—and fabrication can never be perfect."
The researchers leveraged previous research on a low-loss silicon photonic platform to realize their new beam steering system in the visible wavelength — the first chip-scale phased array operating at blue wavelengths using a silicon nitride platform. A major challenge, say the researchers, was working in the blue range, which has the smallest wavelength in the visible spectrum and scatters more than other colors because it travels as shorter, smaller waves.
Another challenge in demonstrating a phased array in blue was that to achieve a wide angle, the researchers had to overcome the challenge of placing emitters half a wavelength apart or at least smaller than a wavelength — 40 nm spacing — which was very difficult to achieve. In addition, in order to make optical phased array useful for practical applications, they needed many emitters. Scaling this up to a large system, they say, would be extremely difficult.
"Not only is this fabrication really hard, but there would also be a lot of optical crosstalk with the waveguides that close," says Shin. "We can’t have independent phase control plus we'd see all the light coupled to each other, not forming a directional beam."
Solving these issues for blue, say the researchers,