Cascaded C+L-band tunable erbium-doped fiber lasers for simultaneous generation of continuous wave and mode-locked pulses

Simultaneous generation of continuous wave (CW) laser and mode-locked pulses employing fiber lasers has not been realized so far to the best of our knowledge. The idea may have various specific applications in the field of photonics and optical communications. Therefore, we propose through numerical simulations a novel method for simultaneous generation of CW laser and train of mode-locked pulses based on two cascaded cavities of Erbium-doped fibers (EDFs) tunable in C+L-band (1555-1625 nm) connected through a fiber Bragg grating (FBG). A single 1480 nm pump is shared for pumping both EDFs by splitting the power between them. Pumping the EDF in first cavity generates a CW laser of linewidth of 0.01 nm tunable in 1555-1625 nm wavelength range using an intra-cavity tunable optical filter (TOF) as well as provides seed that is injected through the FBG for mode-locking in the second cavity. Injecting the seed and pumping into the second cavity generates a CW laser in the second cavity that can be at the same wavelength of the first cavity or at different wavelength in the range between 1555–1625 nm achieved by tuning the center wavelength of a tunable optical bandpass filter (TOBF) in the second cavity. The injected seed is also used in the mode-locking process in the second cavity to produce pulses at the selected wavelength of the TOBF. It is possible to have both CW and pulse train in the second cavity if the TOBF in each cavity has different center wavelength. Else, the first cavity will have CW light while the second cavity generates pulses at same wavelengths. A Mach-Zehnder modulator (MZM) placed inside the second cavity driven by Gaussian pulses at a repetition rate of 2 MHz realizes an active mode-locked fiber laser that generates train of pulses with full-width at half maximum (FWHM) of around 38 ns. The proposed design is optimized for the EDF length and doping concentration of \(\hbox {Er}^{3+}\) in both of the cavities. A wide tuning of 70 nm with average output powers of 22.6 dBm and 24.6 dBm are obtained for the first cavity while average output powers of 21 dBm and 22 dBm are obtained for second cavity for coupling ratios of 10% and 20%, respectively. Optical signal to noise ratio (OSNR) varies from 81.4 to 99 dB for first cavity and from 60 to 87 dB for the second cavity over 1555–1625 nm wavelength range. Finally the effect of increasing the coupling ratio of first cavity on mode-locking process in the second cavity has been investigated. The proposed design of tunable cascaded EDFLs for simultaneous generation of CW laser and train of mode-locked pulses is cost-efficient, simple, and stable permitting a broad tuning of 70 nm in 1555–1625 nm wavelength range.