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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 (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

Launched power is 10 dBm at 1650.9554 nm
Total waveguide power transmission: 𝑒^{−𝛼_𝑤𝑔𝐿}∙𝑒^{−Γ𝛼_𝑔𝑎𝑠𝐶𝐿}, where 𝐶=𝐶_{𝑝𝑝𝑚𝑣}/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):

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

where 𝐶_{𝑝𝑝𝑚,𝑖} is the 𝑖^th measurement sample (𝑁 total) with sampling time 𝑡_{𝑎𝑣𝑒𝑟𝑎𝑔𝑒}

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/𝛼_𝑤𝑔 [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