Narrow linewidth optical source from a femtosecond comb and O-E-O frequency conversion pair

Abstract

Single-longitudinal-mode, narrow-linewidth laser has many applications such as coherent optical communication and long distance optical sensing. One traditional way to generate this laser is using long fiber (e.g. several kilometers or even above) cavity combing stimulated Brillouin scattering (SBS) effect [1]. But this structure exists some intrinsic problems such as being sensitive to environment and too small longitudinal-mode interval, which may degrade the performance (e.g. frequency drifting, mode-hop or multi-longitudinal-mode operation). Another way to generate this laser is based on active feedback control, but a high-stable cavity, a high-precise algorithm and a large-bandwidth feedback loop [2] must be satisfied simultaneously. Moreover, the traditional ways can't tune lasing wavelength in wide waveband. Currently, femtosecond (fs) technology provides a new train of thought. Firstly, the mode-locked fs-laser provide an extremely wideband (20-dB bandwidth of about 100 nm commonly) spectral components, which affords the possibility of wideband wavelength tuning. Secondly, its own mode-lock mechanism grantees the high coherence between every two frequency lines and low absolute phase noise of each frequency line. If using some methods to lock its pulse repetition rate frep and carrier-envelope-offset frequency fceo [3], the fs-laser will upgrade to a stable fs-comb. The linewidth of these comb lines usually reach several Hz or even lower, which is very attractive for narrow-linewidth laser. Thus, a lot of schemes have been proposed to utilize these advantages of fs -comb in single-longitudinal-mode laser. One scheme is extracting an individual line from fs-comb directly based on fiber nonlinearity [4] or injection locking [5]. Another is using fs-comb as a reference source in optical phase locking [6]. In this paper, we propose an O-E-O frequency conversion pair to demonstrate a narrow linewidth optical source from fs-comb. This frequency conversion pair is drove by a DFB laser whose 20-dB linewidth is as large as 920 kHz. By the design of delay match in this pair, the large phase noise of DFB has been highly suppressed. Finally we obtain a low-noise radiation, whose linewidth is only 1/35 of DFB laser.