1. Introduction

In recent years, significant progress has been made on the fabrication of ultrafast fiber lasers (delivering extremely short pulses, in the order of picoseconds or femtoseconds), which have become the key element for multiple applications, such as optical communications, material processing, laser micromachining [1^{[1][2][3][4][5]},2,3,4,5], etc. These reliable, flexible, and compact sources have found applications not only in industrial processes but also in the medical (biological photonics [6,7,8]^[6][7][8]) or military (radar systems [9,10]^[9][10]) fields. In order to generate laser pulses, two different techniques are applied, namely Q-switching or mode-locking operation, both based on the insertion of a variable attenuator, e.g., a saturable absorber (SA), within the laser resonator cavity [11,12]^[11][12]. For both techniques, we can distinguish between active or passive operation modes, with passive approaches presenting advantages in terms of simplicity, low cost, and stability. Several nonlinear elements have been tested as variable attenuators, such as semiconductor-based SAs [13^[13][14],14], carbon nanotubes ^[15], or nonlinear polarization-rotators ^[16], among others. Because of their low saturation intensity and extensive modulation depth, semiconductor SA elements are widely used in commercial lasers ^[17]. However, semiconductor SAs have also demonstrated some limitations, such as narrow working bandwidth and low damage threshold. Therefore, new fabrication methods should be explored to counteract these effects, since they are critical to implementing high-power, ultrafast pulsed lasers [18,19]^[18][19]. It is known, for instance, that semiconductor SAs are highly dependent on structural defects, such as impurities, vacancies, or grain boundaries [20,21,22,23]^{[20][21][22][23]}. It is therefore remarkable to improve the crystal quality as a way of enhancing its optical performance in terms of the absorption band, modulation depth, or saturation intensity [24,25,26]^[24][25][26].

Among semiconductor SAs, indium nitride (InN) has demonstrated remarkable advantages in comparison to other semiconductors, due to its low bandgap energy (0.65–0.9 eV) and the possibility of extending the emission wavelength from infrared to ultraviolet region when using InN and its alloys with GaN or AlN [27,28]^[27][28]. Furthermore, InN epitaxial layers have demonstrated high thermal stability and huge nonlinear behavior at telecom wavelengths [29,30]^[29][30]. For this material, the reduction of the residual carrier concentration is a challenge, since structural defects and incorporation of common impurities like oxygen or hydrogen results in efficient n-type doping ^[28]. Recent progress in molecular beam epitaxy has led to InN epitaxial layers with a residual carrier concentration below 10¹⁸ cm⁻³, which results in an absorption edge energy below 0.7 eV at room temperature ^[30]. It is hence interesting to study this material as a saturable absorber at 1560 nm.

2. Pulsed Laser Operation

The InN saturable absorbers were inserted in a home-built, Er-doped, fiber ring laser cavity for mode-locked operation, as depicted in Figure 1a. 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 ps²/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 ps²/m). It should be noticed that due to the bulk structure and small thickness, small Fabry–Pérot oscillations in the transmission measurement were measured for the InN semiconductor, 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]^[31][32], with a net dispersion coefficient of −0.21 ps², operating in the anomalous dispersion regime.

Figure 1. (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.

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]^[33].

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]^[32]. Figure 1b 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]^[34]. 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 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]. In order to better understand the saturable absorber properties of the proposed InN materials, those properties have been compared to the best performance in our previous work (sample S0′) ^[29].

Inserting sample S1, an ultrafast, mode-locking generation was obtained for an intracavity pump power higher than 35 mW. The maximum output power of the laser cavity (minimum attenuation) was 31.6 mW, which corresponded to a peak power and pulse energy of 25.5 kW and 5.6 nJ, respectively, at an intracavity pump power of 73.7 mW. Figure 2a,b shows the autocorrelation trace and the optical spectrum, which yields a pulse duration of 156.3 fs in the autocorrelation trace (fitted to a Gaussian pulse shape and measured as the full width at half-maximum (FWHM) of the optical pulse, i.e., τpulse=τAC×0.648=156.3fs, where τ_AC = 221 fs is the pulse duration of the autocorrelation function), and there is a 3 dB spectral bandwidth of 26.4 nm at 1564 nm. A 0.648 constant has been applied to transform the autocorrelation (AC) temporal width to the pulse profile, as in reference [44]^[35].

Figure 2. (a) Mode-locked autocorrelation trace for reference sample (S0′) and modified saturable absorbers (SAs) (S1 and S2) with a Gaussian fitting function, i.e., the pulse duration of the autocorrelation function (τ_AC) = 235 fs for S0′, τ_AC = 221 fs for S1, and τ_AC = 190 fs for S2. (b) Optical spectrum of each sample centered in 1560 nm and fitted to a Gaussian function (22.6 nm for reference sample S0′, 25.4 nm for S1, and 40 nm for S2), with a pump power of 70 mW within the laser cavity.

This corresponds to a time–bandwidth product (TBP; measured as Δτ·Δυ, where τ is the pulse duration and υ the optical frequency) of 0.49. This implies that the pulse is not in the transform-limited situation, i.e., the optical pulses are slightly chirped. No Kelly sidebands have been observed in the optical spectrum, which denotes the weak intracavity nonlinearity and dispersion of the fiber laser [42]^[32].

By using S2, a readily highly stable pulsed laser was obtained when the intracavity pump power was higher than 38.5 mW. In the case of maximum pump power (70 mW), the output pulse had an optical power of 30 mW, developing a peak power of 28.2 kW and a 5.4 nJ pulse energy. Concerning the pulse duration of the autocorrelation function, it was estimated at 134.4 fs with a Gaussian fit (τpulse=τAC×0.648=134.4 fs where τ_AC = 190 fs), and the optical bandwidth at 3 dB decay was 40 nm, centered at a wavelength of 1569 nm. In this case, a more chirped pulse was obtained with TBP = 0.62.

Therefore, a reduction of the temporal duration was obtained for samples S1 and S2 concerning reference sample S0′ (166.2 fs), while the optical bandwidth increased from 22.6 nm (S0′) to 40 nm (S2). Also, Figure 4b shows a redshift of the central wavelength in S1 (1564 nm) and S2 (1569 nm) for the reference sample S0′ (1560 nm) was shown, which can be explained by the enhancement of the saturable absorption coefficient, and thus to the reduction of the carrier concentration by means of the higher control of the growth conditions during the fabrication process.

3. Conclusions

In summary, reducing the residual carrier concentration in InN below 10¹⁸ cm⁻³ results in an enhancement of the nonlinear absorption at 1560 nm. Thus, we have demonstrated an InN epitaxial layer, with a thickness of 780 nm, that displays a saturable intensity of 4.4 GW/cm², a modulation depth of 17.3%, and a nonlinear absorption coefficient in up to −3 × 10⁴ cm/GW, and which has been observed attaining an enhanced nonlinear change in transmittance (715% of transmission change). This suggests promising results as saturable absorbers in ultrafast lasers. Based on this saturable absorber, an ultrafast fiber laser was developed with a pulse duration of 134.4 fs and peak power up to 28.2 kW. The InN-based SESAM has demonstrated not only some advantages in terms of ease of fabrication, robustness, and crystalline quality, but also a higher achievable peak power in comparison with other materials. These results make InN a promising candidate for the production of commercial saturable absorbers in the telecom spectral range.