2017 Fiscal Year Research-status Report
Development of Ge-based mid-infrared waveguide platform with strong optical confinement for on-chip gas sensing
| Project/Area Number |
17K14670
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| Research Institution | Tokyo City University |
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Principal Investigator |
徐 学俊 東京都市大学, 工学部, 講師 (80593334)
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| Project Period (FY) |
2017-04-01 – 2019-03-31
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| Keywords | Mid-infrared / Germanium / Waveguide / Grating coupler |
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| Outline of Annual Research Achievements |
In the first year, growth condition for high quality Ge on Si substrate has been established and fundamental MIR photonic devices, including waveguides and fiber-to-waveguide grating couplers (GCs) have been demonstrated.
A two-step growth method has been developed to grow single crystalline Ge on Si substrate. Post annealing at 650 degree for 10 minutes was found to be able to significantly reduce the MIR absorption coefficient at 3.27 um, down to less than 10 cm-1. Based on optimized Ge-on-Si substrate, straight waveguides and fiber-to-waveguide grating couplers were fabricated by optimized process conditions. A transmission measurement system for MIR waveguide devices has then been built based on a 3.27 um DFB laser, TEC-cooled InSb photodetector, ZBLAN single mode fibers, and several other MIR optical components. Successful transmission of MIR laser through the waveguide was observed. The total transmission loss, including propagation loss of waveguide and coupling loss of two fiber-to-waveguide grating couplers, was ~30 dB. The maximum coupling efficiency was ~2.6%, which is much lower than the simulation value (10.7%), which could be attributed to the fact that the polarization of laser in the single-mode fiber was not purely TE. To further increase the coupling efficiency, a free-standing GC and waveguide with subwavelength grating cladding was proposed and designed. Coupling efficiency of about 44% could be achieved through simulation. These achievement lays a solid base for realization of more complex MIR waveguide devices and gas sensors.
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| Current Status of Research Progress |
Current Status of Research Progress
2: Research has progressed on the whole more than it was originally planned.
Reason
The research project is progressing smoothly, almost as planned, since the experimental guideline from material growth to device fabrication and characterization has been well established. High quality Ge-on-Si with low absorption in MIR has been grown, providing a good material platform for the subsequent device fabrication. Waveguide coupling measurement system in the MIR has been built so that necessary photonic devices for gas sensing can be fully characterized. The fabrication process of the MIR photonic devices has also been well optimized. The preliminary measurement results also validated the feasibility of proposed research plan and method, and represented the first demonstration of fiber-to-waveguide grating couplers in the 3.27 μm region. These progresses will accelerate the demonstration of more complex MIR photonic devices and systems for gas sensing experiment in the next year.
Due to the low coupling efficiency of currently designed grating coupler, the output signal from the device was too weak for spectra measurement. The resonant spectra of microdisk resonators were not measured yet. However, the improved grating coupler structure, with a 4 times enhanced coupling efficiency, will solve this problem and it will become much easier to characterize microdisk resonators.
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| Strategy for Future Research Activity |
In the second year, the improved grating coupler and waveguide structures will be fabricated and characterized. Much higher output signal intensity can be expected. Based on this architecture, high Q-factor waveguide-coupled microdisk resonators and high speed thermo-optical modulators will be fabricated and characterized, as the key devices for gas sensing.
The test apparatus for gas sensors will be then built, with a customized gas chamber equipped with vacuum pumps, gas inlets/outlets, mass flow meters, temperature controlled sample stage, MIR-transparent window and electrical connectors. Optical path for in-coupling and out-coupling of MIR laser will also be built. CH4 diluted in N2 atmosphere will be used as detection trace gas.
The sensitivity and signal-noise-ratio of gas sensor will be firstly analyzed with respect to the measured performance of Ge microdisks. Especially, Ge thickness, disk radius, and coupling coefficient between bus waveguide and microdisk, will be optimized to achieve large confinement factor of evanescent wave in air and critical coupling with which maximum sensitivity can be obtained. Photonic integrated circuits for sensing, including grating couplers, modulators and waveguide-coupled microdisks, will then be fabricated. Quantitative sensing experiments of CH4 gas concentration will be finally performed through detection of transmittance change at resonant wavelength.
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| Causes of Carryover |
The reason for the incurring amount to be used next fiscal year is that, some optical components in the MIR waveguide measurement system were used ones and obtained with lower prices.
The amount will be used for building the gas sensing experiment system by combining with the next year's budget.
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