We designed and implemented a compact silicon nitride mode-spot converter that is non-linear and stepwise segmented. The mode-spot converter with a length of 20 μm and a simulated transmission efficiency of 95.7% was obtained using the three-dimensional finite-difference time-domain method and particle swarm optimization algorithm. This device can connect a 10-μm wide waveguide with a 1-μm wide single-mode waveguide for O-band operation. Compared with a linear taper with equivalent transmission efficiency, this device reduces the footprint by 80%. The mode-spot converter was fabricated on a silicon nitride platform using electron-beam lithography and inductively coupled plasma etching processes. The fabricated device exhibits a measured average insertion loss of 0.30 dB in the O-band and 1 dB bandwidth of more than 100 nm.
We propose and experimentally demonstrate a bidirectional angled multimode interferometer (Bi-AMMI) for interleaved wavelength division multiplexing applications. The spectral response of the 2 × 4 Bi-AMMI is obtained using an eigenmode expansion method, including the cross coupling between the input and output ports at the same side of the multimode waveguide. To verify our design, both the standalone and interleaved Bi-AMMIs were fabricated and measured. The Bi-AMMI exhibits an insertion loss of 0.99 to 1.21 dB, and the unwanted cross coupling is lower than −20 dB. The interleaved device exhibits a measured optical loss of 4.77 to 5.02 dB, and the average crosstalk is calculated to be −18 dB.
Microring (MR) devices require thermal stabilization to match their intended operating wavelengths. In this work, we present a scheme for thermal stabilization of MR modulators utilizing integrated temperature sensor and closed-loop control unit based on CMOS integrated circuit. Temperature of the MR modulator is directly monitored by a diode based monolithic temperature sensor. The measured temperature is used in a feedback loop to adjust the thermal tuner of the MR. Thermal steady state and transient simulations are performed showing that temperature tuning range from 30 to 100℃ can be achieved. Considering the relatively slow heat transfer process, unity-gain bandwidth of the control circuit is optimized to ensure loop stability. This design can effectively compensate ambient thermal noise and adjust the deviation of the operating wavelength caused by the process error
In this work, a bidirectional grating coupler for perfectly vertical coupling with enhanced coupling efficiency is proposed. A silicon nitride layer above the grating regain is utilized to enhance the coupling efficiency. With the help of the silicon nitride layer, reflection back into the fiber is diminished and maximal coupling into the guided mode is achieved. In addition, this grating coupler shows strong fiber misalignment tolerance. Genetic algorithm (GA) is used to simultaneously optimize the grating and silicon nitride layer. The optimal design obtained from GA shows that the total in-plane optical coupling in C-band is enhanced from about 56.3% to 67%; meanwhile the back-reflection is reduced from 17.6% to 5.3%. What’s more, the device proposed here shows a wide-band character with a 1-dB-bandwidth of 54 nm. Such a design can provide an efficient and cost-effective solution for optical vertical coupling of a WDM application and low-cost fiber packaging for silicon PIC.
An electrically tunable grating coupler is designed and numerically demonstrated. With a lateral p-i-n diode embedded, the optical spectrum of coupling efficiency can be tuned with the applied voltage. To simulate the coupling spectra response with bias voltage, the optical simulation and electrical simulation are carried out with the commercial software Lumerical Finite-Difference Time-Domain Solutions and Synopsys Sentaurus TCAD. Due to the dual effect of spectrum shift and optical loss, the coupling efficiency spectrum can be greatly modulated. With a bias voltage of 2 V, the resulting spectrum shift is 47.5 nm and the peak coupling efficiency at the designed wavelength center can be modulated from 52% to 10%. In addition, the electrical tuning can be used for compensation of postassembly spectrum shift. The effects of the incident angle error and epoxy curing process are discussed. According to our simulation results, tuning voltages of 1 and 2 V are enough to compensate for the incident angle error of 2.5 deg and 3.5 deg, respectively. For the spectrum shift caused by epoxy bonding, the required tuning voltage is as low as 0.82 V. Though it brings additional optical loss, the tuning technique shows interesting prospects in postassembly coupling optimization or channel equalization.
In this work, a fiber-to-chip optical interface with four output ports is proposed. External lights irradiate vertically from single mode fiber to the center of optical interface can be coupled into silicon photonic chips and split into four siliconon- insulator (SOI) waveguides. If the light is circular polarized, the power of light will be equally split into four ports. Meanwhile, all lights travel in the four channel will be converted into TE polarization. The optical interface is based on a two-dimensional grating coupler with carefully designed duty cycle and period. Simulation results show that the coupling efficiency of each port can reach 11.6% so that the total coupling efficiency of the interface is 46.4%. And Lights coupled into four waveguides are all converted into TE polarization. Further, the optical interface has a simple grating structure allowing for easy fabrication.
The integration of optical devices and electronic integrated circuits (IC) is a main issue for optoelectronic convergence.
In this work, a CMOS post-backend process flow is proposed to potentially achieve a 3-D monolithic optoelectronic
integrated chip. The proposed integrated chip is composed of an IC die as electronic layer and a waveguide device layer
as photonic layer above electronic layer. The photonic layer is fabricated by CMOS post-backend process with a
temperature blow 450 ºC, which would do no harm to the performance of the CMOS ICs. We also fabricated Si3N4
mircoring add-drop filters on a bulk Si wafer. The cross-section of the waveguide is 400 nm × 1 μm, and the radius of
microring is 30μm. Measured results match well with numerical simulations.
A bandpass microwave photonic filter based on an integrated optical signal processor is proposed and demonstrated by numerical simulation. The optical signal processor consisting of double-bus-coupled and series-cascaded silicon microrings (MRs) is used to produce two bandpass responses to process optical carrier signal and sideband signal separately. Because of the tunability of MRs, variable −3 dB bandwidth and tunable operating frequency are achieved. The −3 dB bandwidth and operating frequency can be tuned from 1.5 to 12 GHz and from 15 to 34 GHz, respectively. The loss impact, tuning method, and fabrication error tolerance are also discussed.
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