Abstract
We report the design of an ultra-compact polarization beam splitter with high performance that is based on augmented low-index guiding and subwavelength grating (SWG) structures. The transverse-electric (TE) and transverse-magnetic (TM) modes are confined in high-index silicon (Si) and low-index silicon nitride (Si3N4), respectively. They are separated by using, respectively, a gradually curved Si waveguide and a Si3N4 SWG structure with optimal grating-element. The footprint of the proposed polarization beam splitters (PBS) is 2.9 × 2.25 μm2. The device offers high polarization extinction ratios (PERs) of ~18 dB for the two polarizations, with low insertion losses of ~0.22 dB (~0.71 dB) for the TE (TM) mode at the wavelength of 1550 nm. Over the broad band from λ = 1500–1650 nm, the PERs of the TE and TM modes are above 17 and 16 dB, respectively. By narrowing the operating band to the range from λ = 1500 to 1600 nm, the proposed PBS provides PERs of >17 dB for both polarizations. Finally, the fabrication tolerance of the designed PBS is also addressed and discussed in detail.
Highlights
To achieve the high transmission demands of optical communication systems and construct photonic integrated circuits, devices such as polarization beam splitters (PBSs) and polarization rotators play pivotal roles in manipulating optical signals
According to the guiding principle for an augmented low-index guiding (ALIG) structure18, the Si layer must be thin enough to push most of the power of the TM mode into the Si3N4 layer
We find that the geometrical conditions hSi < 160 nm, hSi3N4 < 1000 nm, and win < 480 nm support only the fundamental TE and TM modes (TE0 and TM0)
Summary
Because of the greater mismatch of the effective indices, it is more difficult for the TE power to be coupled into the TM grating waveguide, as shown, (note that the maximum of the normalized electric field |E| is only 0.16), significantly increasing the PER of the TM mode. It can be seen that the structures [8] and [9] with easy and moderate fabricating difficulties, respectively, obtain smaller footprint than the proposed PBS, the ILs (~3 dB) of [8] are fairly high due to use of copper nanorod and the PERs (~10 dB) of [9] are low due to much overlapping of the two polarizations.
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