Contents I. Introduction
Simultaneous multiwavelength erbium-doped fiber (EDF) laser oscillations along with frequency tunability have been demonstrated using active overlapping linear cavities [1], a high birefringence fiber loop mirror [2], intracavity tunable cascaded long-period fiber gratings [3] or a sampled chirp fiber Bragg grating [4], and a self-seeded Fabry–Pérot laser diode [5]. Because of the relatively large homogeneous gain broadening of EDFs at room temperature, simultaneous multiwavelength lasing in EDF lasers is very sensitive to variations in cavity losses [6]. Homogeneous broadening implies that the gain provided by the EDF at one wavelength uniquely determines the gain at all other wavelengths [7]. Currently reported tunable simultaneous multiwavelength fiber lasers [1] [2] [4] [4] [5], therefore, require careful balancing of cavity losses at each wavelength, particularly if large numbers of wavelengths are to be generated, and if the wavelength is to be tuned within a broad-band wavelength range. Another problem arises if the reported tunable wavelength range is not wide enough for some special applications. Furthermore, these tunable simultaneous multiwavelength fiber lasers [1]–[5] suffer from densely spaced multiple longitudinal modes lying beneath the gain curve because of the long lasing cavity, and this limits their applications as a result in multimode oscillation and a relatively large linewidth [8].
