A gas sensing system includes a signal generator including a wavelength tunable laser, the signal generator providing a first periodic signal and a second periodic signal, wherein the first periodic signal comprises a wavelength scanning signal and the second periodic signal comprises a modulation signal; an optical signal absorption path which is wavelength selective, wherein the generated signal covers at least one of the absorbance band; a signal detector that uses lock-in detection to detect a second harmonic of the second periodic signal after absorption, the signal detector further including a local reference generator, a multiplier, and a low pass filter; a local reference includes a first path (ref1) that outputs sinusoidal signal with frequency equals to that of the second signal in signal generator, and a second path (ref2) that outputs sinusoidal signal of two times ref1 frequency; and a local reference generator having a first phase shifter that is configurable from 0 to 2π and a second phase shifter that shifts 90-degree, wherein the first phase shifter is for an alignment of ref1 with the modulation signal and the second phase shifter provides 90-degree shifts for ref2 from ref1, wherein the first and second paths (ref1 and ref2) are selected by a switch, wherein the switch uses the first path during initialization and the second path for normal operation.