## 3 Discussion  
At both emission wavelengths, the lasers exhibited self-starting stable mode-locking when the diode laser power exceeded a predetermined threshold. We begin with the description of the 1.95 \(\upmu\)m laser. When the laser diode output power reaches 2.5 W, stable mode-locking starts; see Figure 2a. When this pump power is exceeded, the output light train experiences the usual amplitude modulation over the modelocked train. The output light pulse train at 15.6 MHz and the corresponding radio frequency (RF) spectrum centered on this fundamental frequency (corresponding to a resonator length of 6.25 m) are shown in Figure 2a,b respectively. The stability of this mode-locking regime is evidenced by the high extinction ratio in the RF spectrum (65 dB, measured with a resolution bandwidth of 10 Hz). Figure 2c shows the output light pulse in detail. A full width at half maximum (FWHM) measurement of 81 ps identified a secant hyperbolic profile. There is also a ripple at the trailing edge, but this is only a side effect of the photodetector transfer function [34]. Figure 2d shows the output spectrum of this laser, which we also measured. A single peak  
Figure 1: (**a**) Structure of the fiber laser. (**b**) Optical spectra of PM FBG 1 tuned to 1.951 \(\upmu\)m (red dots), and PM FBG 2 tuned to 2.07 \(\upmu\)m (green dots). in the spectrum has a bandwidth of 50 pm at 3 dB, which is approximately the resolution limit of our optical spectrum analyzer (50 pm) [35]. Finally, the measured average output power after the PM isolator was 230 \(\upmu\)W, so the peak power of the pulsed emission is about 100 mW. This corresponds to a laser efficiency of about 0.1%, which is consistent with previous work using TDF [36]. We continue with the characterization of the laser emitting at the optical wavelength of 2.07 \(\upmu\)m. When the output power of the laser diode is 3 W, the laser also exhibits self-starting stable mode-locking; see Figure 3a. At this pump power, the laser delivers one pulse per round trip at a repetition rate of 13.084 MHz, which is the reciprocal of the expected round trip time for a 7.89 m Fabry-Perot resonator. Mode-locking is maintained when the laser diode emits a pump power greater than 3 W, but the laser emits multiple pulses per round trip. For example, when the laser diode output power reaches 4.1 W, two pulses are generated per round trip, and so on until the maximum available pump power is reached. Soliton lasers typically exhibit such behavior. The output sequence of light pulses at 13.084 MHz and the corresponding spectrum from RF, both shown in Figure 3a,b. Another indication of the stability of this mode-locking regime is the high extinction ratio in the RF spectrum (up to 80 dB, measured with a resolution bandwidth of 1 Hz). Figure 3c shows a detailed illustration of the waveform of a single pulse in the time domain. The curve has a FWHM of 94 ps and fits well with a hyperbolic square secant profile (also shown). These output light pulses are slightly wider compared to our earlier finding with emission at 1.95 \(\upmu\)m. As an example, we can compare these results with Ref. [36], where a modelocked fiber laser was presented that uses a TDF as medium gain and an NSA as the mode-locker and is also pumped in the C-band, but with a fiber resonator not polarization-preserved. In this case, the FHWM of the pulse width is considerably shorter (~1.4 ps). This is the result of not using ad hoc filtering within the cavity, except of course for those intrinsically associated with the various elements (couplers, WDMs, etc.). In our case, there is strong filtering by the FBG, see Figure 1a, which reduces the spectral bandwidth of the output pulse while broadening the pulse in the time domain. Since the repetition rate was almost the same, a similar nonlinear parameter is expected but with a larger dispersion, resulting in a wider pulse width in this case.