Multiwavelength pulse generation using an actively mode-locked erbium-doped fiber ring laser based on distributed dispersion cavity

A simple design of a stable multiwavelength pulse generator was demonstrated using a dispersion-tuned actively mode-locked erbium-doped fiber (EDF) ring laser with distributed dispersion cavity. The distributed dispersion cavity in the fiber laser successfully reduced the cross-gain saturation in EDF, and thus, enabled multiwavelength operation. Simultaneous generation of wavelength-tunable 10-GHz pulses up to four different wavelengths was achieved with the same wavelength space of 2.94 nm. The extinction ratio of all wavelengths was above 30 dB. In addition, smooth wavelength tuning was achieved over more than 41 nm when the laser was working at dual-wavelength mode. The super mode noise was ~60 dB below the signal level at both wavelengths. The laser state was found to be very stable

I. Introduction

Stable multiwavelength mode-locked fiber lasers have wide applications in wavelength-division-multiplexing transmission systems, optical signal processing, fiber-optical sensing, optical instrumentation, and microwave photonic systems. The main challenges for erbium-doped fiber (EDF) ring lasers to achieve stable multiwavelength lasing at room temperature are the strong homogeneous line broadening and the cross-gain saturation. Previously, several approaches have been proposed [1]–[4]. Cooling the EDF to 77 K by liquid nitrogen can suppress the homogenous line broadening and the cross-gain saturation [1], but this technique is impractical in many applications. Room-temperature multiwavelength lasing was demonstrated by using multiple gain media in the laser cavity [2] or flattening the gain spectrum [3]. However, these designs are somewhat complex and costly. A novel gain competition suppression method using temporal-spectral multiplexing was put forward by Chen et al. By inserting a pair of nearly identical linearly chirped fiber Bragg gratings before and after EDF, respectively, they realized dual-wavelength actively mode-locked fiber laser [4]. Nevertheless, the two lasing wavelengths are not tunable. As an alternative way, a multiwavelength dispersion-tuned actively mode-locked EDF ring laser has been proposed [5]. A remarkable feature of the technique is that it allows “smooth” wavelength tuning. However, it does not overcome the problem of gain competition. Lee and Shu also suggested a novel scheme for dual-wavelength pulse generation using the dispersion tuning approach [6]. They have produced not only simultaneous but also alternating dual-wavelength tunable pulses. But the configuration is a bit complex and more wavelength lasing is not reported.