Ultra-broadband and low-loss edge coupler for highly efficient second harmonic generation in thin-film lithium niobate

Abstract

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.

Roadmap on nonlinear optics–focus on Chinese research

“…It further paves the way for cavity enhanced nonlinearity for weak light. By designing an ultra-broadband edge coupler and high-performance PPLN nanowaveguides, efficient SHG with a fiber-to-fiber normalized efficiency of 1027%W −1 cm −2 , has recently been demonstrated [105], which is ready for user application (figure 16(b)). A heterostructure cavity composed of a guided-mode resonance (GMR) resonator and distributed Bragg reflectors (figure 16(c)) showcases strongly enhanced electromagnetic field localization to boost SHG by three orders of magnitude [106].…”

Achromatic on-chip focusing of graphene plasmons for spatial inversions of broadband digital optical signals

“…On-chip integrated photonic devices exhibit great potential for applications in many fields, including optical communications, 1 , 2 quantum gates, 3 , 4 signal processing, 5 , 6 and computing 7 , 8 . To fully demonstrate the functionalities of integrated photonic devices, it is essential to manipulate on-chip photonic modes like controlling free-space light beams, such as beam focusing, bending, and steering 9 , 10 .…”

An Ultrabroadband and Cost-Effective Edge Coupler for Efficient Thin Film Lithium Niobate Photonics

“…Vertical couplers, mostly grating couplers [31][32][33][34][35][36][37][38], intrinsically suffer from limited efficiency, limited bandwidth, and polarization sensitivity due to their operational principle. As for edge couplers [39][40][41][42][43][44][45][46][47], taper waveguide structures are usually used in the design and preferred in practical applications, especially in areas with high requirements for both coupling efficiency and bandwidth, such as upconversion single-photon detectors.…”

“…To further improve the coupling efficiency, a coupler consisting of a bilayer taper and a SiON cladding waveguide is proposed that displayed a coupling loss lower than 1 dB/facet in the wavelength range of 1527 nm to 1630 nm [41]. In order to extend the coupling bandwidth to the near-visible band, a coupler comprising a tri-layer taper and a silica cladding waveguide is reported that exhibited a coupling loss of 1 dB/facet and 3 dB/facet at 1550 nm and 775 nm, respectively [44]. Although the performance of the above coupler is outstanding, the fabrication of such a coupler requires high-precision electron-beam lithography, which is expensive and detrimental to wafer-scale fabrication.…”