# High-Quality, InN-Based, Saturable Absorbers for Ultrafast Laser Development
## 3 Pulsed Laser Operation  
The above-described InN saturable absorbers were inserted in a home-built, Er-doped, fiber ring laser cavity for mode-locked operation, as depicted in Figure 3a. The saturable absorber was placed in a free-space region on a reflection configuration. A 300 nm-thick aluminum layer was deposited on the InN film by radio-frequency sputtering at room temperature, to create a semiconductor saturable absorber mirror (SESAM). A commercial, erbium-doped fiber amplifier (EDFA) acted as the gain medium (Accelink, TV series), with a maximum output power of 24 dBm and 16 m of erbium-doped fiber, with a normal group velocity dispersion of 0.016 ps\({}^{2}\)/m. In this experiment, a variable optical attenuator is inserted to control the optical losses within the laser cavity. In order to maximize the optical power onto the sample, a 70/30 optical fiber coupler was included, so that 70% of the signal was recirculated inside the cavity, whereas the remaining 30% was the laser output. This configuration has demonstrated the best results for this type of laser cavity [29], in comparison to a transmitted configuration, as described in previous results [30]. The laser cavity had a total length of 38 m, from which 22 m corresponded to single-mode fiber (SMF) with anomalous dispersion (\(-\)0.021 ps\({}^{2}\)/m). It should be noticed that due to the bulk structure and small thickness, small Fabry-Perot oscillations in the transmission measurement were measured for the InN semiconductor (inset of Figure 1), which yielded a negligible group-velocity dispersion (GVD) coefficient compared to the ones obtained for SMF and EDF. Thus, the laser cavity behaves as a dispersion-managed cavity [41; 42], with a net dispersion coefficient of \(-\)0.21 ps\({}^{2}\), operating in the anomalous dispersion regime. Due to the wurtzite structure of the saturable absorber, no polarization controller was added to the laser cavity, as the material is polarized independently for light impinging along its \(z\)-axis [43]. Therefore, a highly stable, mode-locked, pulsed laser was obtained inside the laser cavity, delivering Gaussian stretched pulses with a repetition rate of 5.6 MHz, as expected for strong dispersion-managed cavities [42]. Figure 3b shows the corresponding oscilloscope trace of the mode-locked pulse train, with a time interval between consecutive pulses of 178.5 ns, which coincides with the optical round trip of the fiber laser cavity working at the fundamental state, i.e., no harmonic generation was observed. The laser cavity length was increased in up to 1.05 km, obtaining Gaussian pulses at the fundamental mode without generating amplified spontaneous emission (ASE) noise from the EDFA. Higher harmonic generation has been detected for longer cavities, modifying the pulse profile at the output, as described in [40]. The corresponding signal-to-noise ratio (SNR) was characterized by a radio frequency (RF) spectrum analyzer, obtaining a value of \(>\)45 dB [29]. No side-peaks were observed  
Figure 3: (**a**) Schematic set-up of the erbium (Er)-doped, mode-locked fiber laser, using InN as a saturable absorber. (**b**) Oscilloscope trace for fundamental mode-locking operations at 5.6 MHz. in the RF spectrum between consecutive pulses, which proves the stability of the laser cavity in the mode-locking regime. An analysis of the laser properties was performed for each sample, as a function of the output power of the laser cavity. For that purpose, the gain power of the EDFA remained constant while varying the linear losses inside the cavity through the variable optical attenuator, and thus, the energy applied to the saturable absorber. In this experiment, a second fiber coupler with a 99/1 ratio, inserted at the output of the laser cavity, monitored the average power and the autocorrelation and spectrum figures for each value of optical power inside the cavity. More details related to the measurement method can be found in [30].