Reivindicaciones

1. A road condition sensor for detecting a state condition of the surface of the roadway in a specific verification region (3) ahead of a vehicle, characterized in that it comprises: - an emitting group (1) for emitting optical pulses (2), comprising: ∘ a first system to generate a first optical frequency comb with a central frequency f1 and a frequencies span of n*Δf1, being n the number of the comb lines and Δf1 the spacing between comb lines, where the spectral bandwidth of the comb covers a region of the spectrum where the refraction index is different if the verification region if in state condition or another: ∘ a second system to generate a second optical frequency comb with a central frequency f2 and a frequencies span of m*Δf2, being m the number of the comb lines and Δf2 the spacing between comb lines, where the spectral bandwidth of the second comb overlaps the spectral bandwidth of the first comb; - a receiving group (5) for a reflected signal (4) comprising: ∘ a photodetector (5.2) intended for receiving the reflected radiation (4) coming from the first and second comb after striking the verification region (3) and outputting an electrical signal S1, with a bandwidth BW; ∘ electronic means to compute the radiofrequency spectrum of signal S1. 2. The road condition sensor according to claim 1, wherein: - the difference of the frequencies of the two combs (|f2-f1|) equals the central frequency bandwidth of the receiving group (BW), - the heterodyne of first comb and second comb generate frequencies with in the bandwidth of the receiving group (BW). 3. The road condition sensor of claims 1 or 2 wherein the dual optical frequency comb is formed by two optical frequency comb generators that use the same laser diode as a source. 4. The road condition sensor of any of the previous claims wherein the optical frequency combs have a central wavelength in a region of the spectrum where the absorption properties of water and ice are different. 5. The road condition sensor of claim 4 wherein the optical frequency combs have a central wavelength in one of the ranges between 900 and 1350 nm, between 1400 and 1800 nm and between 1900 nm and 2100 nm in the near infrared. 6. A method for determining the state condition of the surface of a roadway in a specific verification region (3) ahead of a vehicle, comprising the steps of: - generating a first optical frequency comb with a central frequency f1 and a frequencies span of n*Δf1, being n the number of the comb lines and Δf1 the spacing between comb lines, where the spectral bandwidth of the comb covers a region of the spectrum where the refraction index is different if the verification region if in state condition or another; - generating a second optical frequency comb with a central frequency f2 and a frequencies span of m*Δf2, being m the number of the comb lines and Δf2 the spacing between comb lines, where the spectral bandwidth of the second comb overlaps the spectral bandwidth of the first comb; - combining the output signals of the two combs to obtain a dual comb and directing such a pulse (2) towards the verification region; - receiving a reflected signal (4) at a photodetector (5.2) after striking the verification region (3); - outputting an electrical signal S1, with a bandwidth BW, - computing the radiofrequency spectrum of signal S1, - comparing the amplitude of the line of frequency f = f2-f1 with an amplitude of reference for the pavement in the dry condition; - if f is different than the reference, then, - calculating the ratios of the amplitudes of the frequencies around the central frequency f; - assessing the road condition as "wet" if the ratio is above a threshold and as "ice" if it is below the threshold.