Integrated Photonics

Metasurface has found broad applicability in free-space optics, while its potential to tailor guided waves remains barely explored. By synergizing the Jones matrix model with generalized Snell's law under the phase-matching condition, we propose a universal design strategy for versatile on-chip mode-selective coupling with polarization sensitivity, multiple working wavelengths, and high efficiency concurrently. The coupling direction, operation frequency, and excited mode type can be designed at will for arbitrary incident polarizations, outperforming previous technology that only works for specific polarizations and lacks versatile mode controllability. Here, using silicon-nanoantennas-patterned silicon-nitride photonic waveguides, we numerically demonstrate a set of chip-scale optical couplers around 1.55 μm, including mode-selective directional couplers with high coupling efficiency over 57% and directivity about 23 dB. Polarization and wavelength demultiplexer scenarios are also proposed with 67% maximum efficiency and an extinction ratio of 20 dB. Moreover, a chip-integrated twisted light generator, coupling free-space linear polarization into an optical vortex carrying 1ℏ orbital angular momentum (OAM), is also reported to validate the mode-control flexibility. This comprehensive method may motivate compact wavelength/polarization (de)multiplexers, multifunctional mode converters, on-chip OAM generators for photonic integrated circuits, and high-speed optical telecommunications.

We report on the experimental demonstration of a chip-scale microresonator comb enabled optical frequency synthesizer using an agile and highly-integrated heterodyne optical phase-locked loop with InP-based photonic integrated circuit and commercial-off-the-shelf electronic components. OCIS codes: (250.5300) Photonic integrated circuits; (060.5625) Radio frequency photonics; (060.2840) Heterodyne (140.0140) Lasers and laser optics; (140.3600) Lasers, tunable; (140.3945) Microcavities; (230.5750) Resonators In recent years, chip-scale and low-power optical frequency synthesizers (OFSs) based on self-stabilization of full-octave microresonator combs are being increasingly investigated for various ultra-high-precision applications in which micro-Hertz levels of stability are sought [1]. However, for many practical applications, including optical spectroscopy [2], optical communication [3], light detection and ranging (LiDAR) [4], and some types of frequency metrology, stabilities in the few Hertz range would be very attractive. In this work, we report on the experimental demonstration of a continuously-tunable, microresonator-enabled, ultra-compact OFS near 1550 nm achieved by using a heterodyne optical phase-locked loop (OPLL) [5]. Figure 1(a) illustrates the basic concept of a compact and chip-scale OFS. A microresonator-based optical frequency comb (OFC) was used as an ultra-stable and narrow linewidth source, serving as a master oscillator (MO) [6]. The comb lines are then used as the reference for the heterodyne OPLL. A RF frequency from a tunable RF synthesizer is applied to feedback electronic circuits of the OPLL to introduce a frequency offset, defined by the frequency difference between the master laser and the local oscillator (LO) laser. By tuning the phase section current of the LO laser as well as f RF , the LO is phase-locked to the comb lines. In order for an OFS to synthesize any arbitrary frequency between comb lines, the heterodyne OPLL offset frequency range must be at least half of the comb's free spectra range (FSR). Also for such continuous tuning, the FSR of the comb must be less than the slave laser's mode­hop free tuning range.