Ultra-high extinction ratio polarization beam splitter with extreme skin-depth waveguide

In this Letter, we present a high extinction ratio and compact on-chip polarization beam splitter (PBS), based on an extreme skin-depth (eskid) waveguide. Subwavelength-scale gratings form an effectively anisotropic metamaterial cladding and introduce a large birefringence. The anisotropic dielectric perturbation of the metamaterial cladding suppresses the TE polarization extinction via exceptional coupling, while the large birefringence efficiently cross-couples the TM mode, thus reducing the coupling length. We demonstrated the eskid-PBS on a silicon-on-insulator platform and achieved an ultra-high extinction ratio PBS ($≈<#comment/>60dB$for TE and$≈<#comment/>48dB$for TM) with a compact coupling length ($≈<#comment/>14.5µ<#comment/>m$). The insertion loss is also negligible ($<<#comment/>0.6dB$). The bandwidth is$><#comment/>80$(30) nm for the TE (TM) extinction ratio$><#comment/>20dB$. Our ultra-high extinction ratio PBS is crucial in implementing efficient polarization diversity circuits, especially where a high degree of polarization distinguishability is necessary, such as photonic quantum information processing.

Authors:
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Award ID(s):
Publication Date:
NSF-PAR ID:
10223926
Journal Name:
Optics Letters
Volume:
46
Issue:
9
Page Range or eLocation-ID:
Article No. 2164
ISSN:
0146-9592; OPLEDP
Publisher:
Optical Society of America
1. We present an ultra-broadband silicon photonic polarization beam splitter (PBS) using adiabatically tapered extreme skin-depth (eskid) waveguides. Highly anisotropic metamaterial claddings of the eskid waveguides suppress the crosstalk of transverse-electric (TE) mode, while the large birefringence of the eskid waveguide efficiently cross-couples the transverse-magnetic (TM) mode. Two eskid waveguides are adiabatically tapered to smoothly translate TM mode to the coupled port via mode evolution while keeping the TE mode in the through port. The tapered cross-section of the eskid PBS was designed numerically, achieving a large bandwidth at 1400–1650 nm with extinction ratios$><#comment/>20dB$. We experimentally demonstrated the tapered-eskid PBS and confirmed its broad bandwidth at 1490–1640 nm, limited by laser bandwidth. With its mode evolution, the tapered-eskid PBS is tolerant to fabrication imperfections and should be crucial for controlling polarizations in photonic circuits.
2. We experimentally demonstrate the utilization of adaptive optics (AO) to mitigate intra-group power coupling among linearly polarized (LP) modes in a graded-index few-mode fiber (GI FMF). Generally, in this fiber, the coupling between degenerate modes inside a modal group tends to be stronger than between modes belonging to different groups. In our approach, the coupling inside the$LP11$group can be represented by a combination of orbital-angular-momentum (OAM) modes, such that reducing power coupling in OAM set tends to indicate the capability to reduce the coupling inside the$LP11$group. We employ two output OAM modes$l=+1$and$l=−<#comment/>1$as resultant linear combinations of degenerate$LP11a$and$LP11b$modes inside the$LP11$group of a$∼<#comment/>0.6-km$GI FMF. The power coupling is mitigated by shaping the amplitude and phase of the distorted OAM modes. Each OAM mode carries an independent 20-, 40-, or 100-Gbit/s quadrature-phase-shift-keying data stream. We measure the transmission matrix (TM) in the OAM basis within$LP11$group, which is a subset of the full LP TMmore »
3. The mid-IR spectroscopic properties of$Er3+$doped low-phonon$CsCdCl3$and$CsPbCl3$crystals grown by the Bridgman technique have been investigated. Using optical excitations at$∼<#comment/>800nm$and$∼<#comment/>660nm$, both crystals exhibited IR emissions at$∼<#comment/>1.55$,$∼<#comment/>2.75$,$∼<#comment/>3.5$, and$∼<#comment/>4.5µ<#comment/>m$at room temperature. The mid-IR emission at 4.5 µm, originating from the$4I9/2→<#comment/>4I11/2$transition, showed a long emission lifetime of$∼<#comment/>11.6ms$for$Er3+$doped$CsCdCl3$, whereas$Er3+$doped$CsPbCl3$exhibited a shorter lifetime of$∼<#comment/>1.8ms$. The measured emission lifetimes of the$4I9/2$state were nearly independent of the temperature, indicating a negligibly small nonradiative decay rate through multiphonon relaxation, as predicted by the energy-gap law for low-maximum-phonon energy hosts. The room temperature stimulatedmore »
4. We experimentally demonstrate simultaneous turbulence mitigation and channel demultiplexing in a 200 Gbit/s orbital-angular-momentum (OAM) multiplexed link by adaptive wavefront shaping and diffusing (WSD) the light beams. Different realizations of two emulated turbulence strengths (the Fried parameter$r0=0.4,1.0mm$) are mitigated. The experimental results show the following. (1) Crosstalk between OAM$l=+1$and$l=−<#comment/>1$modes can be reduced by$><#comment/>10.0$and$><#comment/>5.8dB$, respectively, under the weaker turbulence ($r0=1.0mm$); crosstalk is further improved by$><#comment/>17.7$and$><#comment/>19.4dB$, respectively, under most realizations in the stronger turbulence ($r0=0.4mm$). (2) The optical signal-to-noise ratio penalties for the bit error rate performance are measured to be$∼<#comment/>0.7$and$∼<#comment/>1.6dB$under weaker turbulence, while measured to be$∼<#comment/>3.2$and$∼<#comment/>1.8dB$under stronger turbulence for OAM$l=+1$and$l=−<#comment/>1$mode, respectively.
5. We study the relationship between the input phase delays and the output mode orders when using a pixel-array structure fed by multiple single-mode waveguides for tunable orbital-angular-momentum (OAM) beam generation. As an emitter of a free-space OAM beam, the designed structure introduces a transformation function that shapes and coherently combines multiple (e.g., four) equal-amplitude inputs, with the$k$th input carrying a phase delay of$(k−<#comment/>1)Δ<#comment/>φ<#comment/>$. The simulation results show that (1) the generated OAM order ℓ is dependent on the relative phase delay$Δ<#comment/>φ<#comment/>$; (2) the transformation function can be tailored by engineering the structure to support different tunable ranges (e.g., $l={−<#comment/>1},{−<#comment/>1,+1},{−<#comment/>1,0,+1}$, or${−<#comment/>2,−<#comment/>1,+1,+2}$); and (3) multiple independent coaxial OAM beams can be generated by simultaneously feeding the structure with multiple independent beams, such that each beam has its own$Δ<#comment/>φ<#comment/>$value for the four inputs. Moreover, there is a trade-off between the tunable range and the mode purity, bandwidth, and crosstalk, such that the increase of the tunable range leads to (a) decreased mode purity (from 91% to 75% formore »), (b) decreased 3 dB bandwidth of emission efficiency (from 285 nm for$l={−<#comment/>1}$to 122 nm for$l={−<#comment/>2,−<#comment/>1,+1,+2}$), and (c) increased crosstalk within the C-band (from$−<#comment/>23.7$to$−<#comment/>13.2dB$when the tunable range increases from 2 to 4).