Performance study and assessment of phase noise suppression by incoherent addition in a mode-locked fiber laser system Jin Xue ⁎, Nanxi Li, Kan Wu, Jia Haur Wong, Chunmei Ouyang, Sheel Aditya, Perry Ping Shum School of Electrical & Electronic Engineering, Nanyang Technological University, 637553, Singapore abstract article info Article history: Received 21 July 2011 Received in revised form 12 September 2011 Accepted 13 September 2011 Available online 29 September 2011 Keywords: Phase noise Interferometer Fiber laser Mode-locked laser This paper investigates the effectiveness of phase noise suppression by incoherent addition in a passively mode-locked fiber laser. Incoherent addition is achieved by using an interferometer external to the mode- locked laser. Two different types of interferometers, Mach–Zehnder and ring, are investigated experimentally for different background phase noise levels. Measurements show that both types of interferometers can achieve good phase noise reduction for a background phase noise level above -130 dBc/Hz. Effects of disper- sion management and pulse train intensity ratio in the interferometers are also discussed. Multi-stage cas- caded interferometers are proposed for supermode noise suppression of harmonically actively mode- locked lasers. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Mode-locked lasers with their outstanding ultrafast properties have brought remarkable advances in many fields of science and en- gineering, such as optical analog–digital conversion [1], frequency metrology [2] and tomography [3]. Many of these applications require stringent phase noise control of the laser sources. This issue has been widely discussed in the past few decades [4–10]. Although many techniques have been proposed for suppressing phase noise and re- ducing timing jitter [11–20], most of these techniques require com- plex setups either optically or electrically. H. Tsuchida has reported a simple configuration applying external interferometers to reduce the phase noise of semiconductor laser diodes by incoherent intensity addition [21]. Compared to the other methods, this technique has many advantages including simple setup, flexible design, applicable to all types of mode-locked lasers and requiring no change in the source components. In spite of a disadvantage of this technique, namely slight pulse broadening because of incoherently addition of two pulse trains and also because of group velocity dispersion, the technique has a very good potential for realizing low-noise lasers. Tsuchida's method has successfully reduced the phase noise of semiconductor laser diodes which have a relatively high original phase noise. However, it is unknown whether the method is effective for other pulsed sources with lower original phase noise, e.g. other types of solid state lasers and fiber lasers. In this paper, we investigate the noise limiting performance of this method by using a low-noise passively mode-locked fiber laser as the pulsed source and comparing the noise reduction at different original phase noise levels. Two inter- ferometer structures, Mach–Zehnder (MZ) and ring, are compared analytically and experimentally. It is found that for both of the struc- tures, the incoherent addition method can achieve good phase noise reduction by creating deep dips in the spectrum when the original phase noise levels are above -130 dBc/Hz. Influence of dispersion and pulse train intensity ratio in the interferometers is also discussed. Finally, multi-stage cascaded interferometers are proposed as a po- tential solution for supermode noise suppression of harmonically mode-locked lasers. 2. Configuration and analysis To illustrate the basic principle of operation, configurations of MZ interferometer and ring interferometer are shown in Fig. 1(a) and (b), respectively. The MZ interferometer contains a variable delay line (VDL), a variable optical attenuator (VOA), and two couplers. The delay time of the longer path is designed to be much larger than the coherence time of the incident pulse train and to be an integer multi- ple of the pulse repetition period. Therefore the two pulse trains trav- eling in the two arms of the interferometer will coincide with each other at the output coupler to give incoherent intensity addition. Pre- cise optical length control can be realized by finely adjusting VDL. VOA serves to balance the power in the two arms. Assuming that the coupling ratio of the input and the output cou- pler of the MZ interferometer is a:(1 - a) and 50:50, respectively, in Fig. 1(a), the phase noise power spectral density (PSD) S out (f) of the Optics Communications 285 (2012) 153–157 ⁎ Corresponding author. Tel.: + 65 84457230. E-mail address: xuej0005@ntu.edu.sg (J. Xue). 0030-4018/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2011.09.033 Contents lists available at SciVerse ScienceDirect Optics Communications journal homepage: www.elsevier.com/locate/optcom