RSoft Application Gallery Note: Si-Waveguide Absorption-Based Methane Sensor

Tool Used: OptSim Circuit

In this 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 (𝛼_𝑔𝑎𝑠= 0.4347 cm-1)
  • Have the following waveguide parameters:
    • length 𝐿 = 10 cm,
    • intrinsic loss 𝛼_𝑤𝑔 = 2 dB/cm
    • methane mode overlap Γ = 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: 𝑒−𝛼𝑤𝑔𝐿∙𝑒−Γ𝛼𝑔𝑎𝑠𝐶𝐿, where 𝐶=𝐶𝑝𝑝𝑚𝑣/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 (𝑁𝐸𝐴):

OptSim Circuit Allan Deviation Analysis

Indicated blocks produce electrical signal whose average power is the Allan variance in ppmv²:

where 𝐶𝑝𝑝𝑚,𝑖 is the 𝑖th measurement sample (𝑁 total) with sampling time 𝑡𝑎𝑣𝑒𝑟𝑎𝑔𝑒

  • 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 – consistent with assumption that sensor noise is independent of signal power
  • Detection limit optimum at waveguide length = 1/𝛼𝑤𝑔 [see H. Lin et al., “Mid-infrared integrated photonics on silicon: a perspective”, Nanophotonics 7(2), 393-420 (2018).]
  • Sub-10-ppmv detection limit at optimum length for averaging times ≳ 91 seconds
  • Detection limit improves as waveguide loss decreases
  • Results assume sensor noise independent of signal power levels