Typically,　the　performance　of　the　state-of-the-art　laser　sensors　is　insufficient　for　many　high　precision　applications.　Here,　the　authors　report　mode-phase-difference　photothermal　spectroscopy　with　a　dual-mode　anti-resonant　hollow-core　optical　fiber　and　demonstrate　acetylene　detection　with　ultra-high　sensitivity.　Laser　spectroscopy　outperforms　electrochemical　and　semiconductor　gas　sensors　in　selectivity　and　environmental　survivability.　However,　the　performance　of　the　state-of-the-art　laser　sensors　is　still　insufficient　for　many　high　precision　applications.　Here,　we　report　mode-phase-difference　photothermal　spectroscopy　with　a　dual-mode　anti-resonant　hollow-core　optical　fiber　and　demonstrate　all-fiber　gas　(acetylene)　detection　down　to　ppt　(parts-per-trillion)　and　＜1%　instability　over　a　period　of　3　hours.　An　anti-resonant　hollow-core　fiber　could　be　designed　to　transmit　light　signals　over　a　broad　wavelength　range　from　visible　to　infrared,　covering　molecular　absorption　lines　of　many　important　gases.　This　would　enable　multi-component　gas　detection　with　a　single　sensing　element　and　pave　the　way　for　ultra-precision　gas　sensing　for　medical,　environmental　and　industrial　applications.