Generation Of Optical Frequency Comb And Its Applications

A series of equally spaced optical frequencies is referred to as the optical frequency comb. Flat optical frequency comb is a comb where the power of each frequency is equal. Flat optical frequency comb can be generated by phase modulation. Advantages of this technique include the stable optical center frequencies given by the source laser and convenient tuning of the frequency spacing. Flat optical frequency comb is important in both optical communication and measurements. For example, flat optical frequency comb can be used to suppress stimulated Brillion scattering in CATV and improve the performance of optical time domain reflectometer.Strong sinusoidal phase modulation of a CW laser creates multiple sidebands, which can be described as a frequency comb. Such combs are a source of choice for some applications, like WDM system and optical arbitrary waveform generation. These applications need the combs flat and stable.However, by phase modulation alone, the spectral flatness of the comb is so poor that it is out of usable. Some improved schemes have been reported to generate a flat optical frequency comb, such as multiple sinusoidal frequencies modulation, cascaded intensity modulator and phase modulator, and so on. In this paper, we proposal a new method, which use parabolic phase modulation instead of sinusoidal phase modulation, to generate optical frequency comb. Theoretical analysis and unmusical simulation show that the flatness is better than sinusoidal phase modulation, and the flatness and comb number increase with the modulation depth increasing. In the experiment, an optical arbitrary waveform generation is used to generate a parabolic waveform to drive phase modulator. We get OFCs with 3,5,7,9,11,13 comb lines within 2 dB flatness in the experiment, which agrees well with the theoretical analysis.With the generated optical frequency combs, we study some of the applications of the OFC, including the FDM-COTDR based on flat optical frequency comb and the suppression of SBS based on flat optical frequency comb. Experiment results using 13 comb lines demonstrate that compared with conventional COTDR it can bring much quick fading noise reduction capability and a 5.7 dB dynamic range enhancement, and the threshold of SBS can be increased by 10 dB in numerical analysis. Results show that the flat optical combs generated by parabolic phase modulator is usable for frequency division coherent optical time domain reflectometry and suppression for stimulated Brillion scattering.