An ark-shaped diffraction grating is used to selectively reflect light from a laser back into the device. The photo also shows the waveguides used to channel light back and forth. |
Silicon
is an ideal platform for integrated photonic circuits because the
material is cheap and readily available. Silicon chips with an
integrated laser source capable of emitting light at a specific
wavelength are particularly useful in telecommunications. Unfortunately,
silicon is a material with high optical loss, which often degrades the
output power and performance of the laser source. Yongqiang Wei at the
A*STAR Data Storage Institute and co-workers1 have now fabricated a
silicon chip that integrates not only a laser, but also an optical
grating that provides optical gain and ensures that the laser outputs
light at wavelengths near 1,550 nm—the standard operating wavelength for
telecommunications devices.
The
transmission of large amounts of data through an optical fiber is based
on laser beams of different wavelengths that are sent through the fiber
all at the same time. For such multi-channel operation, however, the
lasers need to be tuned to precise wavelengths to avoid cross-talk. This
can be achieved with an optical grating.
So
far, integrating a laser and an optical grating into a silicon chip has
been challenging. The laser is typically made from several thin layers
of different semiconductor materials, while the optical grating itself
is etched out of silicon. Everything has to be precisely aligned, and
the conventional way to achieve this is to grow the laser on a separate
semiconductor chip. “The whole process takes more than 50 steps and
requires the surface roughness of the silicon wafer to be extremely low,
less than 0.3 nanometers,” says Wei.
In
the new device, a light source is placed between a mirror and a curved
optical grating (pictured). The grating acts like a selective mirror
that only reflects light at a specific wavelength back into the laser.
This creates an optical cavity that only allows lasing action at a
specific wavelength, providing the precision necessary for
telecommunications applications. The researchers tested their device and
found that it has good performance, emitting light with optical power
of 2.3 mW—about the same power as a laser pointer—at a highly specific
wavelength.
“The
integration of multiple lasers and optical gratings on a single chip
will be our next challenge,” says Wei. “Also, for practical
applications, we plan to scale up our single-wavelength lasers by
utilizing the same grating structure for a broader range of wavelengths
in order to integrate multiple light sources on the chip.”
The
new device marks a major step toward the realization of commercial
telecommunications devices integrated on a single silicon chip.
The A*STAR-affiliated researchers contributing to this research are from the Data Storage Institute.
