Broadband on-chip polarization mode splitters in lithium niobate integrated adiabatic couplers

Abstract

We report, to the best of our knowledge, the first broadband polarization mode splitter (PMS) based on the adiabatic light passage mechanism in the lithium niobate (LiNbO 3 ) waveguide platform. A broad bandwidth of ~140 nm spanning telecom S, C, and L bands at polarization-extinction ratios (PER) of >20 dB and >18 dB for the TE and TM polarization modes, respectively, is found in a five-waveguide adiabatic coupler scheme whose structure is optimized by an adiabaticity engineering process in titanium-diffused LiNbO 3 waveguides. When the five-waveguide PMS is integrated with a three-waveguide "shortcut to adiabaticity" structure, we realize a broadband, high splitting-ratio (η c ) mode splitter for spatial separation of TE-(H-) polarized pump (700-850 nm for η c >99%), TM-(V-) polarized signal (1510-1630 nm for η c >97%), and TE-(H-) polarized idler (1480-1650 nm for η c >97%) modes. Such a unique integrated-optical device is of potential for facilitating the onchip implementation of a pump-filtered, broadband tunable entangled quantum-state generator.

Broadband Biphoton Generation and Polarization Splitting in a Monolithic AlGaAs Chip

“…We see that the number of events measured in case (a) represents 85% of the total number of emitted pairs, demonstrating the high efficiency of the polarization splitting region design. This efficiency is limited by the variation of the splitting ratio across the biphoton bandwidth, which can be reduced by the use of adiabatic couplers such as the ones described in ref .…”

“…38 On the other hand, the design of the polarization splitting region could be refined by using adiabatic couplers which, in LN waveguides without an on-chip photon pair source, have been shown to exhibit a flat spectral profile with splitting ratios above 98% over a spectral range of more than 100 nm. 19 Another interesting approach could be the use of inverse design 39 to improve the figure-of-merit for the directional coupler, as done for example in ref 40. In any case, the device presented in this work can already be used as a source of broadband frequency anticorrelated photon pairs separated in two different spatial modes, directly employable for information processing protocols based on high-dimensional quantum states. 41,42 Using both input arms of the splitting region to generate orthogonally polarized photon pairs would also allow to obtain a source of polarization entangled photon pairs directly separated in two spatial modes, as reported in ppLN, 12 albeit with a narrow bandwidth (0.25 nm) in the latter work.…”

“…Indeed, on the one hand polarization is one of the most used degrees of freedom in quantum information protocols, widely employed both for deterministic separation of photons or to encode entanglement; on the other hand, broadband frequency states for high-dimensional encoding are attracting a growing interest thanks to their capability to convey large-scale quantum information into a single spatial mode, an important asset for scalability. Although broadband integrated polarization mode splitters working in the classical regime have been demonstrated in Si-based platforms, − polymers and lithium niobate waveguide circuits, to date, there exists no device capable of combining the generation of broadband biphoton states and their deterministic splitting through polarization. Moreover, to combine these two very distinct functionalities in a simple monolithic design that requires only very few fabrication steps is a significant technological challenge.…”

Polarization beam splitter on lithium niobate on insulator using a silicon nitride assisted waveguide

“…ADCs have been shown to be a promising PBS structure on SOI 17 because of a large birefringence of the waveguide on SOI. But there are fewer PBSs 18 , 19 on the lithium niobate on insulator (LNOI) compared with SOI because of a smaller waveguide birefringence, which leads to a worse performance for the same structure. Perhaps the reason is that the SOI platform has a stronger structure birefringence compared with the LNOI platform, and the material birefringence of lithium niobate (LN) can be managed to compensate for the structure birefringence 20 .…”

Compact, Broadband and Low-Loss Polarization Beam Splitter on Lithium-Niobate-On-Insulator Using a Silicon Nanowire Assisted Waveguide

“…However, there are lack of PBS designs on LNOI, making it difficult to control the polarizations. Recently, the adiabatic coupler [8] and Mach-Zehnder interferometer [9] have been used to realized ultra-broadband PBSs on the LNOI platform. However, these PBSs have large footprints, limiting their applications in ultra-compact PICs.…”