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Si-Waveguide Absorption-Based Methane Sensor
Tool Used: OptSim Circuit
In this OptSim Circuit application note, we will demonstrate an Absorption-based methane sensor using Si waveguide. This methane sensor will:
- Probe the 1650.96-nm line in methane absorption spectrum (๐ผ_๐๐๐ = 0.4347 cm-1)
- Have the following waveguide parameters:
- length ๐ฟ = 10 cm,
- intrinsic loss ๐ผ_๐ค๐ = 2 dB/cm
- methane mode overlap ฮ = 25.5%
- Demonstrate 8-dB input/output coupling
- Demonstrate a measured detection limit of 100 ppmv for 60-second averaging time
OptSim Circuit Cppmv Measurement
- Launched power is 10 dBm at 1650.9554 nm
- Total waveguide power transmission: ๐โ๐ผ๐ค๐๐ฟโ๐โฮ๐ผ๐๐๐ ๐ถ๐ฟ, where ๐ถ=๐ถ๐๐๐๐ฃ/106
- Receiver noise, assumed to be independent of input signal level, calibrated for 100-ppmv detection limit with 60-second averaging time
- Receiver calibration based on calculated time-averaged noise-equivalent power (NEP) and absorbance (๐๐ธ๐ด):
OptSim Circuit Allan Deviation Analysis
Indicated blocks produce electrical signal whose average power is the Allan variance in ppmvยฒ:
where ๐ถ๐๐๐,๐ is the ๐th measurement sample (๐ total) with sampling time ๐ก๐๐ฃ๐๐๐๐๐
- Allan deviation (ppmv) versus averaging time matches results from Tombez et al. for 10-dBm launch power
- Detection limit improves (worsens) for higher (lower) launch power โ consistent with assumption that sensor noise is independent of signal power
- Detection limit optimum at waveguide length = 1/๐ผ๐ค๐ [see H. Lin et al., โMid-infrared integrated photonics on silicon: a perspectiveโ, Nanophotonics 7(2), 393-420 (2018).]
- Sub-10-ppmv detection limit at optimum length for averaging times โณ 91 seconds
- Detection limit improves as waveguide loss decreases
- Results assume sensor noise independent of signal power levels