Optical antennas are key components in optical phased arrays for light detection and ranging technology requiring long sensing range and high scanning resolution. To achieve a narrow beam width in the far-field region, antenna lengths of several millimeters or more are required. To date, such long antennas have been impossible to achieve in silicon waveguides because currently demonstrated technologies do not allow accurate control of grating strength. Here, we report on a new type of surface-emitting silicon waveguide with a dramatically increased antenna length of L = 3.65 m m . This is achieved by using a subwavelength metamaterial waveguide core evanescently coupled with radiative segments laterally separated from the core. This results in a far-field diffracted beam width of 0.025°, which is a record small beam divergence for a silicon photonics surface-emitting device. We also demonstrate that by using a design with L -shaped surface-emitting segments, the radiation efficiency of the antenna can be substantially increased compared to a conventional design, with an efficiency of 72% at the wavelength of 1550 nm.
Spectral filters are important building blocks for many applications in integrated photonics, including datacom and telecom, optical signal processing and astrophotonics. Sidewall-corrugated waveguide grating is typically the preferred option to implement spectral filters in integrated photonic devices. However, in the high-index contrast silicon-on-insulator (SOI) platform, designs with corrugation sizes of only a few tens of nanometers are often required, which hinders their fabrication. In this work, we propose a novel geometry to design complex Bragg filters with an arbitrary spectral response in silicon waveguides with laterally coupled Bragg loading segments. The waveguide core is designed to operate with a delocalized mode field, which helps reduce sensitivity to fabrication errors and increase accuracy on synthesized coupling coefficients and the corresponding spectral shape control. We present an efficient design strategy, based on the layer-peeling and layer-adding algorithms, that allows to readily synthesize an arbitrary target spectrum for our cladding-modulated Bragg gratings. The proposed filter concept and design methodology are validated by designing and experimentally demonstrating a complex spectral filter in an SOI platform, with 20 non-uniformly spaced spectral notches with a 3-dB linewidth as small as 210 pm.
Recent developments of photonic integrated circuits for the mid-infrared band has opened up a new field of attractive applications for group IV photonics. Grating couplers, formed as diffractive structures on the chip surface, are key components for input and output coupling in integrated photonic platforms. While near-infrared optical fibers exhibit large mode field diameters compared to the wavelength, in the long-wave regime commercially available single-mode optical fibers have mode field diameters of the order of the operating wavelength. Consequently, an efficient fiber-chip surface coupler designed for the long-wave infrared range must radiate the power propagating in the waveguide with a higher radiation strength than a conventional grating coupler in the near-infrared range. In this article, we leverage the short electrical length required for long-wave infrared couplers to design a broadband all-dielectric micro-antenna for a suspended germanium platform at 7.67 µm. The design methodology is inspired by fundamental grating coupler equations, which remain valid even when the micro-antenna has only two or three diffractive elements. A simulated coupling efficiency of ~ 40% is achieved with a 1-dB bandwidth broader than 430 nm, which is almost twice the typical fractional bandwidth of a conventional grating coupler. In addition, the proposed design is markedly tolerant to fiber tilt misalignments of ±10°. This all-dielectric micro-antenna design paves the way for efficient fiber-chip coupling in long-wavelength midinfrared integrated platforms.
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