46$\,\times\,$2.5 GHz Mode-Locked Erbium-Doped Fiber Laser With 25-GHz Spacing

We demonstrate a mode-locked, multiwavelength erbium-doped fiber laser at 2.5 GHz with 25-GHz channel spacing and that is wavelength tunable from 1545 to 1558 nm. Stable, room-temperature operation is obtained by exploiting four-wave mixing in a length of highly nonlinear fiber. At a central wavelength of 1549.5 nm, we obtain 30 wavelengths within 3 dB and 46 wavelengths within 6 dB; the full-width at half-maximum of the envelope is 85 ps.

I. Introduction

Optical sources that generate pulses at multiple wavelengths are important for applications including fiber optic sensing, instrumentation, and communications. Erbium-doped fiber (EDF) lasers are ideal for generating such pulses. In order to obtain stable, multiwavelength operation with narrow wavelength spacing, approaches such as cooling the fiber to 77°K, temporal multiplexing, cascaded or composite cavities, frequency-shifting, and four-wave mixing (FWM) in a length of highly nonlinear fiber (HNLF), are used to overcome the limitations associated with homogeneous broadening of the EDF gain medium [1]–[12]. For continuous-wave (CW) operation, an EDF laser emitting 24 wavelengths with a spacing of 100 GHz has been achieved based on FWM in a length of HNLF [8] and 70 wavelengths with a spacing as narrow as 25 GHz has been obtained using an inhomogenous loss mechanism [9]. A mode-locked EDF laser operating on more than 24 wavelengths with 50-GHz spacing based on frequency-shifting was reported in [4]. FWM has also been considered for mode-locked operation [10]–[12]; however, the number of wavelengths is usually limited (up to 8 in [12]).