A gain switched dual comb spectroscopy device for spectroscopically detecting a sample substance, comprising a first slave laser light source configured to generate a first frequency comb having a first comb free spectral range, and a second slave laser light source configured to generate a second frequency comb having a second comb free spectral range which is different from the first comb free spectral range. A master laser light source is configured to inject seed light in to both slave laser light sources therewith to injection lock the generation of both the first and the second frequency combs. A photodetector part is arranged to combine the first and the second frequency combs and to detect a beat signal the spectrum of which comprises a third frequency comb including beat tones produced by interference between the combined first and second frequency combs.