Cold atoms have important applications in high-precision metrology, sensing, navigation, etc. Laser cooling is crucial to acquire cold atoms[1-2]. In particular, 780 nm narrow linewidth laser is a key enabling factor for cold Rb atoms. The linewidth of the 780nm laser should be below 1 MHz to exactly match the energy levels of Rb atoms. This far exceeds the linewidth limit of Fabry-Perot (FP) lasers. Distributed Feedback (DFB) lasers exhibit much narrower linewidths compared to FP lasers[3]. However, the period of the grating for a 780nm DFB laser is much smaller than the period of the gratings of DFB lasers working at longer wavelength, e.g. at telecommunication wavelength. This presents a significant challenge in manufacturing 780nm DFB lasers[4]. In this work we designed a 780nm DFB laser having a sidewall grating with a period of 360nm, a duty cycle of 50%, an etching depth of 550nm, and a ridge waveguide width of 1.2μm. The laser’s epitaxial layer’s fundamental properties were verified by a 780nm FP laser, with a 110mW single-sided output power. The DFB’s grating was fabricated by electron-beam lithography (EBL) and Inductively Coupled Plasma (ICP) etching. Ultimately, our goal is to achieve a DFB laser with a linewidth below 1 MHz.
With the blooming of artificial intelligence technology, the distant sensing systems with an agile deployment, a high refreshing rate and a high precision are demanded to map the fast-changing environment in terms of 3D point cloud for autonomous vehicles and smart home products. Frequency modulated continuous wave (FMCW), as a coherent detection method, shows a high precision, a high selectivity against ambient light, the intrinsic capability in speed sensing and eye-safety. However, FMCW usually needs a discrete Fourier transform to resolve the distance from the beating between the transmitted and received signals, leading to limitation on the acquisition rate and higher requirement in the frequency chirping span, speed, and linearity of the FMCW laser. Here we demonstrate an electro-optic comb-enabled absolute distance ranging technology with an acquisition rate up to 10 kHz and a precision better than 100 mm. We show that real-time distance resolving can be achieved with only two adjacent interference peaks among many peaks generated over one modulation frequency chirping period, boosting the acquisition rate of LiDAR system for ambiguity-free distance measurement.
Thin-film lithium niobate is a promising material platform for integrated nonlinear photonics, due to its high refractive index contrast with the excellent optical properties. However, the high refractive index contrast and correspondingly small mode field diameter limit the attainable coupling between the waveguide and fiber. In second harmonic generation processes, lack of efficient fiber-chip coupling schemes covering both the fundamental and second harmonic wavelengths has greatly limited the overall efficiency. We design and fabricate an ultra-broadband tri-layer edge coupler with a high coupling efficiency. The coupler allows efficient coupling of 1 dB / facet at 1550 nm and 3 dB / facet at 775 nm. This enables us to achieve an ultrahigh overall second harmonic generation normalized efficiency (fiber-to-fiber) of 1027 % W − 1 cm − 2 (on-chip second harmonic efficiency ∼3256 % W − 1 cm − 2) in a 5-mm-long periodically-poled lithium niobate waveguide, which is two to three orders of magnitude higher than that in state-of-the-art devices.
Integrated photonics provides a route to both miniaturization of quantum key distribution (QKD) devices and enhancing their performance. A key element for achieving discrete-variable QKD is a single-photon detector. It is highly desirable to integrate detectors onto a photonic chip to enable the realization of practical and scalable quantum networks. We realize a heterogeneously integrated, superconducting silicon-photonic chip. Harnessing the unique high-speed feature of our optical waveguide-integrated superconducting detector, we perform the first optimal Bell-state measurement (BSM) of time-bin encoded qubits generated from two independent lasers. The optimal BSM enables an increased key rate of measurement-device-independent QKD (MDI-QKD), which is immune to all attacks against the detection system and hence provides the basis for a QKD network with untrusted relays. Together with the time-multiplexed technique, we have enhanced the sifted key rate by almost one order of magnitude. With a 125-MHz clock rate, we obtain a secure key rate of 6.166 kbps over 24.0 dB loss, which is comparable to the state-of-the-art MDI-QKD experimental results with a GHz clock rate. Combined with integrated QKD transmitters, a scalable, chip-based, and cost-effective QKD network should become realizable in the near future.
A flat-top and low-crosstalk 4-channel wavelength-division multiplexer based on LN thin film is designed. By utilizing narrow straight directional couplers, the crosstalk is <-10 dB and -3 dB bandwidths are ~20 nm for all channels
We report on light trapping by a moving refractive index front inside a silicon waveguide, the so-called optical push broom effect. The front generated by a fast pump pulse collects and traps the energy of a signal wave with smaller group velocity tuned near to the band gap of the waveguide with hyperbolic dispersion. The energy of the signal wave is accumulated inside the front and distributed in frequency. The presented effect can be utilized to compress signals in time and space.
We present thermally tunable silicon coupled resonators optical waveguide (CROW) with an ultralow tuning power of 1.26mW per free spectral range by exploiting thermal isolation trenches close to the ring waveguides. When the micro-rings space is 300μm, the thermal crosstalk is 1.05 pm.
Recently the developments of high contrast optics, such as high contrast grating (HCG), have attracted much attention.
Much of the existing work has been focused on structures that can be characterized as ‘Cartesian’, i.e., which are easily
described by functions that are separable in the Cartesian coordinates. Yet optical fields with cylindrical rather than
Cartesian symmetries, such as Laguerre-Gaussian (LG) modes and their relatives including both the scalar LG modes
and cylindrical vectorial (CV) modes, can be more efficiently manipulated by high contrast structures that have the same
kind of cylindrical symmetries, hence best described in a polar or cylindrical coordinate. An example of such a structure
is the angular grating based silicon photonics micro-ring optical vortex emitter device we reported.
An efficient treatment of cylindrical high contrast structures requires the decomposition of Fourier components of both
the field and the structure in the cylindrical coordinates, so that the coupling process between the field Fourier
components via the structure can be calculated. We have implemented a semi-analytical model that fully describes the
3D vectorial coupling process using a transverse spatial Fourier analysis in the cylindrical space. This model can deal
with HCG structures in cylindrical coordinates with high precision and fast speed, enabling rapid yet accurate simulation
of the coupling of planar waveguide modes with optical vortex modes carrying photonic orbital angular momenta and
allowing optimization of the emission coefficient and emitted beam quality. The details of the method and optimized
silicon photonics integrated OAM emitter devices will be presented.
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