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Multi-wavelength on-chip coupling of visible light

By Robert Niffenegger, David Reens [Lincoln Laboratory, Massachusetts Institute of Technology]
Jules Stuart, John Chiaverini [Lincoln Laboratory and Massachusetts Institute of Technology]
C oupling multiple wavelengths light coupling to PICs that enables use in light to the interface. Moreover, the smaller

of visible light into photonic-
single-mode waveguides make alignment
integrated-circuits (PICs) demanding environments. mode sizes of blue light in optical fiber and
with robust packaging will be critical for Background for low-loss coupling more difficult—
an expanding set of applications from Because most of the applications submicron tolerances are routinely required,
quantum information processing to biology. mentioned above can rely on external laser leading to more stringent specifications
Optogenetics, for instance, requires light sources, fiber coupling of laser light into for initial alignment and maintenance of
near the blue end of the visible spectrum PIC chips is the most typical approach for that alignment. The latter concern is an
to stimulate neuronal activity in living current and near-term devices and systems. issue, especially in the situations relevant
tissue and is poised to capitalize on PIC (Extremely low size, weight, and power for quantum information processing (QIP)
capabilities in implantable substrates [1]. quantum sensing applications may be an applications where extremes of temperature
Data communication in the visible spectrum exception, but even these applications and/or pressure must be endured. Because of
can lead to higher bandwidth transmission, will see initial demonstrations with fiber- these differences when compared to silicon-
and direct connection to chips can enable coupled PICs.) Coupling of light from photonics, approaches to packaging must
inter-core communication. Phased-array fiber to silicon-based PICs is a mature also be different to allow operation in the
emitter geometries based on PICs are at technology, but significant challenges arise new visible regime.
the threshold of new display modalities. when applying the same techniques at Here we focus on the use of on-chip
Quantum-information technologies also wavelengths lower than the typical telecom visible-light coupling for trapped-ion QIP
utilize visible light in several different bands (i.e., wavelengths around 1550nm). [3], though we believe the techniques
promising physical implementations, and While many of the same design principles described here are broadly applicable to
photonic integration is key to achieving and structures can be modified for use in the many of the applications described above. In
practical applications for large-scale visible spectrum, the materials systems and this technology, individual atomic ions are
quantum computing, communication, tighter tolerances that are required in this held above the surface of a chip via radio-
and sensing [2]. Each of these disciplines frequency range render many of the usual frequency (RF) and direct current (DC)
has its own use cases—several being in packaging techniques ineffective. Chief fields produced by electrodes patterned on
atypical environments for microelectronics among these is the unfortunate fact that the chip. The chip is maintained in ultra-
and electro-optics, such as in vivo, in attachment methods employing adhesives high vacuum (UHV) to reduce background
vacuo, at low temperatures, and on moving at the fiber-chip interface fail for even pressure that can limit the lifetime of trapped
platforms. It is, therefore, paramount to moderate-intensity blue light because of ions. Laser beams are focused onto the
develop methodologies for robust coupling “yellowing” of the adhesive over time. This ions to prepare, manipulate, and read out
of light across the visible spectrum into yellowing process results in even so-called their electronic states, which can be long-
PICs to continue advancement in these “ultraviolet (UV)-compatible” epoxies lived and have relatively long quantum
areas. Here we describe a potentially becoming opaque to blue light—in some coherence times. Recently developed PICs
broadly applicable technique for visible- cases this occurs seconds after application of for laser-beam addressing of ions utilize

Figure 1: a) (left) Multiple wavelengths of light edge-coupled into a photonic integrated circuit (PIC) via a fiber-array block. Waveguides route the light to diffractive
grating couplers that focus the light on ions trapped above the surface of the chip (trap electrodes not shown); b) (right) Observing red light coupling via glass fiber array.
The chip is 1cm on a side. Inset: detail of red light coupled into the PIC waveguide on chip (top view). The glass fiber array is on the right and red light is visible in an on-
chip waveguide on the left.

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