Ultra-flat optical frequency comb generation based on a polarization modulator and a Butterworth band-stop filter

A new, to the best of our knowledge, scheme for the generation of ultra-flat optical frequency combs (OFCs) is proposed, which mainly uses a polarization modulator (PolM) and a first-order Butterworth band-stop filter. The PolM is driven by a digital periodic square-wave signal, and the output of the PolM is an uneven OFC. The function of the Butterworth band-stop filter is to obtain flat and stable OFCs by changing its bandwidth. The comb spacing of the OFCs can be adjusted by changing the period of the square-wave signal. The theoretical model for the scheme is also established. The simulation results show that the generator constructed according to the proposed scheme can produce 64, 128, and 256 comb lines with 200 MHz comb spacing and with spectral flatness lower than 0.3 dB. The comb spacing can be further increased by using the square-wave signal at GHz rates, and the generated OFCs exhibit good frequency tunability.