Orthogonal-Phase-Modulated LITES Sensor

Lissajous mode decoupling

A light-induced thermoelastic spectroscopy sensor using orthogonal phase modulation and Lissajous mode decoupling to simultaneously measure two gases on a single quartz tuning fork without time division or extra channels.

Hanxu, Ma · Qiao, Shunda · He, Ying · Sun, Haiyue · Ma, Yufei

Reports on Progress in Physics 2026

Specifications

CH₄ detection limit
{'zh': '0.32', 'en': '0.32'} ppm
C₂H₂ detection limit
{'zh': '0.29', 'en': '0.29'} ppm
CH₄ average relative systematic error
{'zh': '0.54', 'en': '0.54'} %
C₂H₂ average relative systematic error
{'zh': '0.79', 'en': '0.79'} %
CH₄ normalized noise-equivalent absorption coefficient
{'zh': '5.09×10⁻⁹', 'en': '5.09×10⁻⁹'} cm⁻¹·W·Hz⁻¹ᐟ²
C₂H₂ normalized noise-equivalent absorption coefficient
{'zh': '6.18×10⁻⁹', 'en': '6.18×10⁻⁹'} cm⁻¹·W·Hz⁻¹ᐟ²

Advantages

Single channel detects two gases

Two lasers are modulated at the same frequency with a 45° phase difference, combined to co-excite one quartz tuning fork; orthogonal demodulation by a lock-in amplifier yields both gas concentrations simultaneously.

Decoupling suppresses crosstalk

By establishing a two-dimensional forced vibration model of the quartz tuning fork under dual-path orthogonal phase modulation, the vibration mode coupling mechanism was revealed, providing theoretical basis and optimization pathway for suppressing channel crosstalk.

Applications