Sitao Chen

Improved 8-channel silicon mode demultiplexer with grating polarizers

An improved 8-channel silicon mode demultiplexer is realized with TE-type and TM-type grating polarizers at the output ends, and these gratings serve as fiber-chip couplers simultaneously. The present 8-channel silicon mode demultiplexer includes a three-waveguide PBS (for separating the TE0 and TM0 modes) and six cascaded ADCs (for demultiplexing the high-order modes of both polarizations). The grating polarizers with high extinction ratios are used to filter out the polarization crosstalk in the 8-channel hybrid multiplexer efficiently and the measured crosstalk for all the mode-channels of the improved 8-channel mode multiplexer is reduced greatly to ~-20dB in a ~100nm bandwidth.

Low-loss and broadband 2 × 2 silicon thermo-optic Mach–Zehnder switch with bent directional couplers

A low-loss and broadband silicon thermo-optic switch is proposed and demonstrated experimentally by using a Mach-Zehnder Interferometer with 2×2 3 dB power splitters based on bent directional couplers (DCs). The bent DCs are introduced here to replace the traditional 2×2 3 dB power splitters based on multimode interferometers or straight DCs, so that one achieves a coupling ratio of ∼50%∶ 50%, as well as low excess loss over a broadband. The demonstrated Mach-Zehnder switch (MZS) has a ∼140  nm bandwidth for an excess loss of <1  dB and an extinction ratio of >20  dB. The present MZS also shows excellent reproducibility and good fabrication tolerance, which makes it promising for realizing N×N optical switches.

Silicon hybrid demultiplexer with 64 channels for wavelength/mode-division multiplexed on-chip optical interconnects

A monolithically integrated 64-channel hybrid demultiplexer on silicon is demonstrated experimentally to enable wavelength-division-multiplexing and mode-division-multiplexing simultaneously for realizing an ultra-large capacity optical-interconnect link. The present hybrid demultiplexer consists of a four-channel mode multiplexer realized with three cascaded asymmetrical directional-couplers and four identical arrayed-waveguide gratings (AWGs) with 16 channels. For the fabricated hybrid multiplexer, the excess loss and the crosstalk are about -7 and -10  dB, respectively. Better performances can be achieved by minimizing the imperfections (particularly in AWGs) in the fabrication processes. The present hybrid demultiplexer is scalable to have more channels by utilizing more wavelengths, modes, and polarizations.

High-order microring resonators with bent couplers for a box-like filter response

High-order microring resonator (MRR) filters with bent directional couplers are proposed and demonstrated to achieve a box-like filter response. When using bent couplers, the coupling ratio can be adjusted easily by choosing the length of the coupling region, and the excess loss is almost zero while the perimeter of the microring length is unchanged. For the present fabricated five-microring filters with bent directional couplers, the excess loss is less than 1.0 dB, the out-of-band extinction ratio is ∼36  dB, and the response has rising and falling edges as sharp as 48  dB/nm. The thermal tunability of the high-order MRR filter with a Ti-microheater is also demonstrated and the thermally tuning efficiency is about 0.10  nm/mW.

Experimental demonstration of simultaneous mode and polarization-division multiplexing based on silicon densely packed waveguide array.

A silicon mode and polarization-division multiplexing scheme based on a densely packed waveguide array structured as a bus waveguide is introduced. A short adiabatic taper is adopted for (de)multiplexing. Such a structure shows theoretical insertion losses that are <0.05  dB and crosstalk that is <-20  dB over a wide wavelength band for all five supported modes. The structures for (de)multiplexing are fabricated and characterized experimentally. A device, which consists of a multiplexer, a 50-μm-long straight-bus waveguide, and a demultiplexer, exhibits insertion losses that are <0.6  dB and crosstalk that is <-15  dB over an 80 nm wavelength band. The demonstrated (de)multiplexer has a total length of 60 μm, and the bus waveguide has an effective width of 1.58 μm.

Compact monolithically-integrated hybrid (de)multiplexer based on silicon-on-insulator nanowires for PDM-WDM systems.

A compact silicon hybrid (de)multiplexer is designed and demonstrated by integrating a single bi-directional AWG with a polarization diversity circuit, which consists of an ultra-short polarization-beam splitter (PBS) based on a bent coupler and a polarization rotator (PR) based on a silicon-on-insulator nanowire with a cut corner. The present hybrid (de)multiplexer can operate for both TE- and TM- polarizations and thus is available for PDM-WDM systems. An 18-channel hybrid (de)multiplexer is realized with 9 wavelengths as an example. The wavelength-channel spacing is 400GHz (i.e., Δλ(ch) = 3.2nm) and the footprint of the device is about 530μm × 210μm. The channel crosstalk is about -13dB and the total excess loss is about 7dB. The excess loss increases by about 1~2dB due to the cascaded polarization diversity circuit in comparison with a single bi-directional AWG.

Compact Dense Wavelength-Division (De)multiplexer Utilizing a Bidirectional Arrayed-Waveguide Grating Integrated With a Mach–Zehnder Interferometer

A compact wavelength-division (de)multiplexer is proposed and demonstrated experimentally to achieve doubled channel number and halved channel spacing by utilizing a bidirectional arrayed-waveguide grating integrated with an Mach-Zehnder interferometer optical interleaver. As an example, an 18-channel wavelength-division (de)multiplexer with a channel spacing of 200 GHz is designed and fabricated. The measured excess loss is about 8 dB and the channel crosstalk is -15 ~ -18 dB. The footprint of this fabricated (de)multiplexer is about 520 μm × 190 μm.

Compact Eight-Channel Thermally Reconfigurable Optical Add/Drop Multiplexers on Silicon

A compact thermally reconfigurable optical add/ drop multiplexer (ROADM) is realized with 500 nm × 220 nm silicon-on-insulator (SOI) strip waveguides. The demonstrated ROADM has eight channels with a channel spacing of 400 GHz and consists of two identical arrayed-waveguide gratings integrated with eight thermo-optic switches on the same chip. These thermo-optic switches have a broad bandwidth with low loss (<;1 dB) as well as high extinction ratio (>20 dB) for covering all wavelength channels by utilizing point-symmetric Mach-Zehnder interferometers with bent directional couplers. The ROADMs footprint is about 770 μm × 880 μm only due to the ultra-strong confinement ability of SOI strip waveguides. The total on-chip loss of the fabricated ROADM is ~15 and ~7 dB for the transmitted signals and the added/dropped signals, respectively.

Cascaded Ring-Resonators for Multi-Channel Optical Sensing With Reduced Temperature Sensitivity

A configuration with cascaded ring-resonators is designed and demonstrated for realizing multi-channel optical sensing with reduced temperature sensitivity. The present configuration integrates a microring-based 1 x N wavelength-selective power splitter and N microring transducers. The 1 x N wavelength-selective power splitter serves as a 1 x N power splitter and N bandpass optical filters, which helps realize a compact chip with an optical sensor array. As an example, a sensor array with three microring transducers is demonstrated by using silicon-on-insulator nanowires. The present multi-channel optical sensor tolerates a temperature variation within ~10 °C.

Polarization-selective microring resonators

Polarization-selective microring resonators (MRRs) are designed and demonstrated to work with resonances for only one of TE and TM polarizations for the first time, which can be used for realizing the crossing-free, compact and scalable multi-wavelength coherent receiver with dual polarizations. The present polarization-selective MRRs are realized by manipulating the polarization-dependence of the bending loss of the micro-resonators and the coupling ratio of the bent directional couplers introduced here. The demonstrated TM-type MRR and the TE-type MRR work well as a wavelength-selective optical filter for TM polarization and TE polarization, respectively, as designed. On the other hand, the resonance of the undesired orthogonal polarization mode is depressed significantly with very high extinction ratio. For the realized TM-type MRR, the peak-transmission at the drop port is depressed to be lower than -50dB and the loss of the transmission at the through port is very low (<0.5dB) when TE polarization mode is input. For the realized TE-type MRR, the peak-transmission at the drop port is depressed to be ~-40dB and the loss of the transmission at the through port is very low (<0.5dB) when TM-polarization mode is input. The present polarization-selective TM- and TE-type MRRs are useful for the applications with multiple wavelengths and dual polarizations.