The wavelength modulation indices are measured based on harmonic amplitude ratio of 4famp/6famp (4fampand 6famp are the 4- and 6-th harmonic central peak amplitudes correspondingly) with the Doppler-freeselective reflection modulation spectroscopy. The experiments for the 6S1/2(F = 4) → 6P3/2(F′ = 5)transition of cesium D2 line with 30-MHz linewidth were carried out. The 4f- and 6f-harmonic signalswere detected with two digital lock-in amplifiers separately. The maximum error for modulation indicesmeasurement was ±0.1 within the range of m from 3 to 6. The non-linear modulation behaviour of anexternal cavity diode laser induced by voltage tuning was studied with this method. The method formodulation indices measurement does not require a solid etalon as usual for measuring the wavelengthmodulation depth and the absorption linewidth correspondingly.
