A source module for a dual-comb spectrometer comprising: a first semiconductor laser source operable to output a first light signal containing a first frequency comb consisting of a finite series of discrete frequencies separated by a first intermode beat frequency ( f rep,1); a first electrical driver connected to the first semiconductor laser source to apply a first drive current signal to cause generation of the first light signal; a second semiconductor laser source operable to output a second light signal containing a second frequency comb consisting of a finite series of discrete frequencies separated by a second intermode beat frequency ( f rep,2) having a frequency offset from the first intermode beat frequency; a second electrical driver connected to the second semiconductor laser source to apply a second drive current signal to cause generation of the second light signal; an electrical output line from the first semiconductor laser source arranged to extract an AC modulation signal from the first semiconductor laser source that is generated by an internal light field of the first semiconductor laser source oscillating at the first intermode beat frequency ( f rep,1); an electrical reference generator operable to output a reference signal having a reference frequency (Δ f rep); and a frequency locking circuit arranged to combine the reference signal with the AC modulation signal extracted from the first semiconductor laser source so that the second intermode beat frequency ( f rep,2) of the second light signal is locked at a frequency offset equal to the reference frequency (Δ f rep) from the first intermode beat frequency ( f rep,1) of the first light signal, thereby to phase lock the second semiconductor laser source in one degree of freedom as a follower to the first semiconductor laser source which acts as its master, wherein the frequency locking circuit is a phase-locked loop comprising an oscillator controlled to generate an AC output signal at an output frequency offset by the reference frequency (Δf rep) from the first intermode beat frequency ( f rep,1), control of the oscillator being effected by a first electrical mixer arranged to mix the AC output signal from the oscilllator with the AC modulation signal and thereby generate a difference signal at a difference frequency ( f rep,1 - f rep,2), and a second electrical mixer arranged to mix the difference signal with the reference signal and thereby generate an error signal, the error signal being fed back to the oscillator to lock its output frequency to said output frequency. a first semiconductor laser source operable to output a first light signal containing a first frequency comb consisting of a finite series of discrete frequencies separated by a first intermode beat frequency ( f rep,1); a first electrical driver connected to the first semiconductor laser source to apply a first drive current signal to cause generation of the first light signal; a second semiconductor laser source operable to output a second light signal containing a second frequency comb consisting of a finite series of discrete frequencies separated by a second intermode beat frequency ( f rep,2) having a frequency offset from the first intermode beat frequency; a second electrical driver connected to the second semiconductor laser source to apply a second drive current signal to cause generation of the second light signal; an electrical output line from the first semiconductor laser source arranged to extract an AC modulation signal from the first semiconductor laser source that is generated by an internal light field of the first semiconductor laser source oscillating at the first intermode beat frequency ( f rep,1); an electrical reference generator operable to output a reference signal having a reference frequency (Δ f rep); and a frequency locking circuit arranged to combine the reference signal with the AC modulation signal extracted from the first semiconductor laser source so that the second intermode beat frequency ( f rep,2) of the second light signal is locked at a frequency offset equal to the reference frequency (Δ f rep) from the first intermode beat frequency ( f rep,1) of the first light signal, thereby to phase lock the second semiconductor laser source in one degree of freedom as a follower to the first semiconductor laser source which acts as its master, wherein the frequency locking circuit is a phase-locked loop comprising an oscillator controlled to generate an AC output signal at an output frequency offset by the reference frequency (Δf rep) from the first intermode beat frequency ( f rep,1), control of the oscillator being effected by a first electrical mixer arranged to mix the AC output signal from the oscilllator with the AC modulation signal and thereby generate a difference signal at a difference frequency ( f rep,1 - f rep,2), and a second electrical mixer arranged to mix the difference signal with the reference signal and thereby generate an error signal, the error signal being fed back to the oscillator to lock its output frequency to said output frequency.