Versatile Mode-Locked Operations in an Er-Doped Fiber Laser with a Film-Type Indium Tin Oxide Saturable Absorber

We demonstrate the generation of versatile mode-locked operations in an Er-doped fiber laser with an indium tin oxide (ITO) saturable absorber (SA). As an epsilon-near-zero material, ITO has been only used to fashion a mode-locked fiber laser as an ITO nanoparticle-polyvinyl alcohol SA. However, this type of SA cannot work at high power or ensure that the SA materials can be transmitted by the light. Thus, we covered the end face of a fiber with a uniform ITO film using the radio frequency magnetron sputtering technology to fabricate a novel ITO SA. Using this new type of SA, single-wavelength pulses, dual-wavelength pulses, and triple-wavelength multi-pulses were achieved easily. The pulse durations of these mode-locked operations were 1.67, 6.91, and 1 ns, respectively. At the dual-wavelength mode-locked state, the fiber laser could achieve an output power of 2.91 mW and a pulse energy of 1.48 nJ. This study reveals that such a proposed film-type ITO SA has excellent nonlinear absorption properties, which can promote the application of ITO film for ultrafast photonics.


Introduction
Mode-locked fiber lasers are vital for modern research and applications, e.g., for optical communication, radar systems, and material processing [1][2][3]. A saturable absorber (SA) is the most important device in a mode-locked fiber laser, which can emit ultra-short pulses. Various soliton phenomena could be obtained using SAs fabricated by diverse materials. Additionally, multi-wavelength fiber lasers have attracted much research interest for wide application in ultrafast optics, optical communications, biomedical research, and radar systems. Using the SAs with high nonlinearity in the fiber laser cavity, the multi-wavelength pulses could be achieved [4][5][6][7]. Accordingly, the fabrication of novel and high performance SAs from various two-dimensional (2D) materials have become a popular field in ultrafast laser research.
Recently, 2D materials have been investigated as SAs for fashioning pulsed fiber lasers because of the outstanding saturable absorption properties [8]. Single-wall carbon nanotube (SWCNT) is an ideal material for SAs due to its various properties like broadband saturable absorption, ultrafast recovery The fiber was vertically fixed on a stable pedestal to ensure that the ITO film could be uniformly deposited on its end face. The distance from substrate to target was 90 mm. The chamber was pumped to 2 × 10 −5 Torr, and the automatic sputtering process was performed in Ar (99.99%) at a flow rate of 30 sccm. After 5 min of sputtering at a rf sputtering power of 100 W, the film-type ITO SA was prepared.

Characterization
After the ITO SA was prepared, the scanning electron microscope (SEM) (Sigma 500, ZEISS, Oberkochen, Germany) image of the end face of fiber head was investigated, as shown in Figure 1a. The red circle shows the position of the fiber core. The SEM image of the ITO film at the end face of the fiber core is shown as the inset in the Figure 1a. The film consisted of ITO NPs about 15 nm in size. Obviously, the whole end face was covered with a smooth and large-scale ITO film, so the light could be transmitted though the SA material. To characterize the roughness and the thickness of the ITO film, an atomic force microscope (AFM) (Multimode 8, Bruker, Germany) was employed. The ITO film was deposited on a SiO 2 substrate under the same conditions. The end face of the fiber core that was used in this experiment was made of SiO 2 , so the parameters (including the thickness, the roughness, and so on) of the ITO film deposited on the SiO 2 substrate should be the same as the ITO film deposited on the fiber core. According to the 3-dimentional (3D) AFM image, as shown in Figure 1b, the roughness of the ITO film was Ra 0.73, suggesting that our ITO film was quite smooth. The AFM image is shown in Figure 1c, and the corresponding height measurement is displayed in Figure 1d. The thickness of the ITO film investigated was 60 nm. By controlling the sputtering time, the thickness can be changed easily for different requirements. Nanomaterials 2019, 9, x FOR PEER REVIEW 3 of 11 deposited on its end face. The distance from substrate to target was 90 mm. The chamber was pumped to 2 × 10 −5 Torr, and the automatic sputtering process was performed in Ar (99.99%) at a flow rate of 30 sccm. After 5 min of sputtering at a rf sputtering power of 100 W, the film-type ITO SA was prepared.

Characterization
After the ITO SA was prepared, the scanning electron microscope (SEM) (Sigma 500, ZEISS, Oberkochen, Germany) image of the end face of fiber head was investigated, as shown in Figure 1a. The red circle shows the position of the fiber core. The SEM image of the ITO film at the end face of the fiber core is shown as the inset in the Figure 1a. The film consisted of ITO NPs about 15 nm in size. Obviously, the whole end face was covered with a smooth and large-scale ITO film, so the light could be transmitted though the SA material. To characterize the roughness and the thickness of the ITO film, an atomic force microscope (AFM) (Multimode 8, Bruker, Germany) was employed. The ITO film was deposited on a SiO2 substrate under the same conditions. The end face of the fiber core that was used in this experiment was made of SiO2, so the parameters (including the thickness, the roughness, and so on) of the ITO film deposited on the SiO2 substrate should be the same as the ITO film deposited on the fiber core. According to the 3-dimentional (3D) AFM image, as shown in Figure  1b, the roughness of the ITO film was Ra 0.73, suggesting that our ITO film was quite smooth. The AFM image is shown in Figure 1c, and the corresponding height measurement is displayed in Figure  1d. The thickness of the ITO film investigated was 60 nm. By controlling the sputtering time, the thickness can be changed easily for different requirements.   The crystal structure of ITO film was tested using an X-Ray diffraction (XRD)(D8 Advance, Bruker, Billerica, MA, USA). Some diffraction peaks corresponding to the (211), (222), (400), (411), (440), (611), and (622) planes in ITO are clearly observed in Figure 2a, respectively, which indicate that the ITO film is of high-quality. To analysis the elemental composition of our sample, the energy dispersive spectrometer (EDS) (QUANTAX EDS, Bruker, Germany) spectrum of the ITO film was investigated, as shown in Figure 2b. The peaks attributed to In and Sn are observed. Moreover, the atomic ratio of In to Sn, which is 91.83:8.17, is shown inset in Figure 2b. It is noteworthy that compared with the target, the atomic ratio of ITO film is slightly misaligned. This may because that a few Sn atoms were not smoothly deposited on the SiO 2 during the deposition process. As a result, the position of the absorption peak may have a red shift compared to that from the ITO film with an In/Sn molar ratio of 90:10 [53]. The crystal structure of ITO film was tested using an X-Ray diffraction (XRD)(D8 Advance, Bruker, Billerica, MA, USA). Some diffraction peaks corresponding to the (211), (222), (400), (411), (440), (611), and (622) planes in ITO are clearly observed in Figure 2a, respectively, which indicate that the ITO film is of high-quality. To analysis the elemental composition of our sample, the energy dispersive spectrometer (EDS) (QUANTAX EDS, Bruker, Germany) spectrum of the ITO film was investigated, as shown in Figure 2b. The peaks attributed to In and Sn are observed. Moreover, the atomic ratio of In to Sn, which is 91.83:8.17, is shown inset in Figure 2b. It is noteworthy that compared with the target, the atomic ratio of ITO film is slightly misaligned. This may because that a few Sn atoms were not smoothly deposited on the SiO2 during the deposition process. As a result, the position of the absorption peak may have a red shift compared to that from the ITO film with an In/Sn molar ratio of 90:10 [53]. The linear absorption spectrum of ITO film was measured with a UV/vis/NIR spectrophotometer (U-4100, Hitachi, Tokyo, Japan), as shown in Figure 3a. The detection light sources were a xenon lamp (for the UV region) and a tungsten halogen lamp (for the vis/NIR region) with a mW-level-power. Obviously, the absorbance increased with the wavelength. At 1558 nm, the absorbance was measured as 4.8%. The absorption bandwidth of ITO film was quite wide in the NIR, allowing for its application of photonics devices in the NIR. Additionally, the nonlinear optical absorption was investigated using a typical balanced twin-detector method. The pump source used here was a home-made nonlinear polarization rotation mode-locked EDFL with 560 fs pulses at the wavelength of 1560 nm and a repetition rate of 33.6 MHz. The recorded results are shown in Figure 3b. The transmittance shows a nonlinear dependence on the pump power, and the saturable absorption property of the ITO film can be obtained by analyzing this relation. By fitting the experimental results, the modulation depth and saturation intensity were calculated, which were 8.3% and 11.9 MW/cm 2 , respectively. The linear absorption spectrum of ITO film was measured with a UV/vis/NIR spectrophotometer (U-4100, Hitachi, Tokyo, Japan), as shown in Figure 3a. The detection light sources were a xenon lamp (for the UV region) and a tungsten halogen lamp (for the vis/NIR region) with a mW-level-power. Obviously, the absorbance increased with the wavelength. At 1558 nm, the absorbance was measured as 4.8%. The absorption bandwidth of ITO film was quite wide in the NIR, allowing for its application of photonics devices in the NIR. Additionally, the nonlinear optical absorption was investigated using a typical balanced twin-detector method. The pump source used here was a home-made nonlinear polarization rotation mode-locked EDFL with 560 fs pulses at the wavelength of 1560 nm and a repetition rate of 33.6 MHz. The recorded results are shown in Figure 3b. The transmittance shows a nonlinear dependence on the pump power, and the saturable absorption property of the ITO film can be obtained by analyzing this relation. By fitting the experimental results, the modulation depth and saturation intensity were calculated, which were 8.3% and 11.9 MW/cm 2 , respectively. The crystal structure of ITO film was tested using an X-Ray diffraction (XRD)(D8 Advance, Bruker, Billerica, MA, USA). Some diffraction peaks corresponding to the (211), (222), (400), (411), (440), (611), and (622) planes in ITO are clearly observed in Figure 2a, respectively, which indicate that the ITO film is of high-quality. To analysis the elemental composition of our sample, the energy dispersive spectrometer (EDS) (QUANTAX EDS, Bruker, Germany) spectrum of the ITO film was investigated, as shown in Figure 2b. The peaks attributed to In and Sn are observed. Moreover, the atomic ratio of In to Sn, which is 91.83:8.17, is shown inset in Figure 2b. It is noteworthy that compared with the target, the atomic ratio of ITO film is slightly misaligned. This may because that a few Sn atoms were not smoothly deposited on the SiO2 during the deposition process. As a result, the position of the absorption peak may have a red shift compared to that from the ITO film with an In/Sn molar ratio of 90:10 [53]. The linear absorption spectrum of ITO film was measured with a UV/vis/NIR spectrophotometer (U-4100, Hitachi, Tokyo, Japan), as shown in Figure 3a. The detection light sources were a xenon lamp (for the UV region) and a tungsten halogen lamp (for the vis/NIR region) with a mW-level-power. Obviously, the absorbance increased with the wavelength. At 1558 nm, the absorbance was measured as 4.8%. The absorption bandwidth of ITO film was quite wide in the NIR, allowing for its application of photonics devices in the NIR. Additionally, the nonlinear optical absorption was investigated using a typical balanced twin-detector method. The pump source used here was a home-made nonlinear polarization rotation mode-locked EDFL with 560 fs pulses at the wavelength of 1560 nm and a repetition rate of 33.6 MHz. The recorded results are shown in Figure 3b. The transmittance shows a nonlinear dependence on the pump power, and the saturable absorption property of the ITO film can be obtained by analyzing this relation. By fitting the experimental results, the modulation depth and saturation intensity were calculated, which were 8.3% and 11.9 MW/cm 2 , respectively. To highlight the advantages of the proposed ITO film SA prepared by magnetron sputtering deposition technique, we compared it with the PVA-type SA here. Firstly, the light absorption and reflection of PVA would cause an insertion loss in the laser cavity. Using an EDFL at a wavelength of 1560 nm, the insertion loss was experimentally recorded to be 1.4 dB. Compared to the PVA-type SAs (the insertion losses were 5.5, 2.5, 3.5, and 6.1 dB, respectively) [42,[54][55][56], the insertion loss of the ITO film SA was significantly lower. Secondly, the PVA-type SA cannot work with a high power due to the weak thermal capacity of PVA. Without the limitation of PVA, the damage threshold can be significantly increased, offering the possibility to work under high-energy conditions.  To highlight the advantages of the proposed ITO film SA prepared by magnetron sputtering deposition technique, we compared it with the PVA-type SA here. Firstly, the light absorption and reflection of PVA would cause an insertion loss in the laser cavity. Using an EDFL at a wavelength of 1560 nm, the insertion loss was experimentally recorded to be 1.4 dB. Compared to the PVA-type SAs (the insertion losses were 5.5, 2.5, 3.5, and 6.1 dB, respectively) [42,[54][55][56], the insertion loss of the ITO film SA was significantly lower. Secondly, the PVA-type SA cannot work with a high power due to the weak thermal capacity of PVA. Without the limitation of PVA, the damage threshold can be significantly increased, offering the possibility to work under high-energy conditions.

Results and Discussion
In this section, we discuss three mode-locked operations in our EDFL based on the ITO film SA. In our experiments, the mode-locked operation was obtained with a length of the cavity at 104.8 m. It is generally known that self-mode-locked operation can sometimes be obtained with a long-length cavity, so the fiber laser was tested without using ITO SA. By regulating the pump power and PCs carefully, no pulse operation was observed, indicating that there was no self-mode-locked operation in the cavity. Using the ITO film SA in the cavity, single-, dual-, and triple-wavelength pulses were regulating by adjusting the PCs and pump power.

Results and Discussion
In this section, we discuss three mode-locked operations in our EDFL based on the ITO film SA. In our experiments, the mode-locked operation was obtained with a length of the cavity at 104.8 m. It is generally known that self-mode-locked operation can sometimes be obtained with a long-length cavity, so the fiber laser was tested without using ITO SA. By regulating the pump power and PCs carefully, no pulse operation was observed, indicating that there was no self-mode-locked operation in the cavity. Using the ITO film SA in the cavity, single-, dual-, and triple-wavelength pulses were regulating by adjusting the PCs and pump power.

Single-Wavelength Pulses
Mode-locked state was achieved at the pump power of 287 mW by adjusting the two PCs. The output optical spectrum with the central wavelength at 1558.5 nm is provided in Figure 5a. In addition, the 3-dB bandwidth is 0.27 nm. However, the output optical spectrum did not show the Kelly sideband. This proved that the mode-locked operation was not at the conventional soliton mode-locked state. According to the zoom-in single pulse profile, as shown in Figure 5b, the pulse duration is nearly 1.67 ns. The time bandwidth product (TBP) is about 55.7, much larger than the theorical value of 0.315. This reveals that the optical pulse was highly chirped, primarily due to the long length of the laser cavity. Regulating the length of the total cavity is a common strategy to compress the pulse width. However, the long length of the laser cavity contributes to the generation of solitons, so that the length of the cavity cannot be arbitrarily shortened. As shown in Figure 5c,d, the repetition rate was 1.96 MHz, corresponding to the total laser cavity length. The signal-to-noise ratio (SNR) is 40 dB, suggesting the mode-locked operation exhibited a good stability. Moreover, the measurement of the radio frequency (RF) spectrum in a span of 100 MHz further proved the stability of the mode-locked operation (inset in Figure 5d). Nanomaterials 2019, 9, x FOR PEER REVIEW 6 of 11

Single-Wavelength Pulses
Mode-locked state was achieved at the pump power of 287 mW by adjusting the two PCs. The output optical spectrum with the central wavelength at 1558.5 nm is provided in Figure 5a. In addition, the 3-dB bandwidth is 0.27 nm. However, the output optical spectrum did not show the Kelly sideband. This proved that the mode-locked operation was not at the conventional soliton mode-locked state. According to the zoom-in single pulse profile, as shown in Figure 5b, the pulse duration is nearly 1.67 ns. The time bandwidth product (TBP) is about 55.7, much larger than the theorical value of 0.315. This reveals that the optical pulse was highly chirped, primarily due to the long length of the laser cavity. Regulating the length of the total cavity is a common strategy to compress the pulse width. However, the long length of the laser cavity contributes to the generation of solitons, so that the length of the cavity cannot be arbitrarily shortened. As shown in Figure 5c,d, the repetition rate was 1.96 MHz, corresponding to the total laser cavity length. The signal-to-noise ratio (SNR) is 40 dB, suggesting the mode-locked operation exhibited a good stability. Moreover, the measurement of the radio frequency (RF) spectrum in a span of 100 MHz further proved the stability of the mode-locked operation (inset in Figure 5d).

Dual-Wavelength Pulses
At the pump power of 268 mW, the dual-wavelength pulses were achieved easily by regulating the PCs. The output optical spectrum is shown in Figure 6a and contains two wavelengths and centered at 1557.6 and 1558.5 nm. The 3-dB bandwidths of the two wavelengths are 0.130 and 0.331 nm, respectively. A single-pulse, as shown in Figure 6b, has a pulse duration of 6.91 ns. The inset of Figure 6b shows the typical pulse train with a period of 510.2 ns, which matches the roundtrip time of the cavity and verifies the mode-locked operation. It is worth mentioning that the trace of pulses exhibits an amplitude envelope modulation. This phenomenon was due to the power instability and the phonon vibration of the SA material. The RF spectrum, which could test the stability of the modelocked fiber laser, is shown in Figure 6c. The peak observed at the repetition rate of 1.96 MHz has a SNR of 50 dB, suggesting that the mode-locking state exhibits an excellent stability. Moreover, the RF

Dual-Wavelength Pulses
At the pump power of 268 mW, the dual-wavelength pulses were achieved easily by regulating the PCs. The output optical spectrum is shown in Figure 6a and contains two wavelengths and centered at 1557.6 and 1558.5 nm. The 3-dB bandwidths of the two wavelengths are 0.130 and 0.331 nm, respectively. A single-pulse, as shown in Figure 6b, has a pulse duration of 6.91 ns. The inset of Figure 6b shows the typical pulse train with a period of 510.2 ns, which matches the roundtrip time of the cavity and verifies the mode-locked operation. It is worth mentioning that the trace of pulses exhibits an amplitude envelope modulation. This phenomenon was due to the power instability and the phonon vibration of the SA material. The RF spectrum, which could test the stability of the mode-locked fiber laser, is shown in Figure 6c. The peak observed at the repetition rate of 1.96 MHz has a SNR of 50 dB, suggesting that the mode-locking state exhibits an excellent stability. Moreover, Nanomaterials 2019, 9, 701 7 of 11 the RF spectrum was also measured within a wide bandwidth of 100 MHz, as shown in the inset of Figure 6c, which further verified that the mode-locked operation exhibits a high stability. As shown in Figure 7, by fixing the PCs at a dual-wavelength pulses state, the output optical spectra and single pulse versus the pump power were recorded. The central wavelength and the 3-dB bandwidth are nearly unchanged while the pump power increased, as shown in Figure 7a. Figure 7b shows the dependence of the pulse width versus the pump power. The pulse width was fixed at about 6.91 ns, and the intensities were up-regulated while the pump powers were regulated from 268 to 310 mW. The output power was raised from 1.73 to 2.91 mW as the pump power was regulated from 254 to 403 mW. According to the repetition frequency of 1.96 MHz, the single pulse energy increased from 0.89 to 1.48 nJ, as shown in Figure 6d. Nanomaterials 2019, 9, x FOR PEER REVIEW 7 of 11 spectrum was also measured within a wide bandwidth of 100 MHz, as shown in the inset of Figure  6c, which further verified that the mode-locked operation exhibits a high stability. As shown in Figure  7, by fixing the PCs at a dual-wavelength pulses state, the output optical spectra and single pulse versus the pump power were recorded. The central wavelength and the 3-dB bandwidth are nearly unchanged while the pump power increased, as shown in Figure 7a. Figure 7b shows the dependence of the pulse width versus the pump power. The pulse width was fixed at about 6.91 ns, and the intensities were up-regulated while the pump powers were regulated from 268 to 310 mW. The output power was raised from 1.73 to 2.91 mW as the pump power was regulated from 254 to 403 mW. According to the repetition frequency of 1.96 MHz, the single pulse energy increased from 0.89 to 1.48 nJ, as shown in Figure 6d.   spectrum was also measured within a wide bandwidth of 100 MHz, as shown in the inset of Figure  6c, which further verified that the mode-locked operation exhibits a high stability. As shown in Figure  7, by fixing the PCs at a dual-wavelength pulses state, the output optical spectra and single pulse versus the pump power were recorded. The central wavelength and the 3-dB bandwidth are nearly unchanged while the pump power increased, as shown in Figure 7a. Figure 7b shows the dependence of the pulse width versus the pump power. The pulse width was fixed at about 6.91 ns, and the intensities were up-regulated while the pump powers were regulated from 268 to 310 mW. The output power was raised from 1.73 to 2.91 mW as the pump power was regulated from 254 to 403 mW. According to the repetition frequency of 1.96 MHz, the single pulse energy increased from 0.89 to 1.48 nJ, as shown in Figure 6d.

Triple-Wavelength Multi-Pulses
Triple-wavelength multi-pulses were achieved by adjusting the PCs carefully at a higher pump power of~420 mW. According to the analysis of the output optical spectrum, as shown in Figure 8a, Nanomaterials 2019, 9, 701 8 of 11 the central peak of the spectrum was located at 1558.5 nm, which was the same as the main peak from the spectra of the single-and dual-wavelength pulses. The 3-dB bandwidth were all about 0.20 nm. Figure 8b shows the pulse train of the triple-wavelength multi-pulses. The period of the pulses train was 510.2 ns, which implies that the fundamental repetition rate remained 1.96 MHz. The zoomed-in view of a single pulse with a pulse width of about 1 ns, which has a symmetric hyperbolic secant intensity profile, is shown in Figure 8c. Triple-wavelength multi-pulses were achieved by adjusting the PCs carefully at a higher pump power of ~420 mW. According to the analysis of the output optical spectrum, as shown in Figure 8a, the central peak of the spectrum was located at 1558.5 nm, which was the same as the main peak from the spectra of the single-and dual-wavelength pulses. The 3-dB bandwidth were all about 0.20 nm. Figure 8b shows the pulse train of the triple-wavelength multi-pulses. The period of the pulses train was 510.2 ns, which implies that the fundamental repetition rate remained 1.96 MHz. The zoomedin view of a single pulse with a pulse width of about 1 ns, which has a symmetric hyperbolic secant intensity profile, is shown in Figure 8c. In the experiments conducted in this study, it is noteworthy that the multi-wavelength modelocked operations were obtained without comb filter in our fiber laser. The polarization-dependent loss attributed to the PCs and birefringence of the SMF can cause the comb filtering effect. Furthermore, there was a large nonlinearity in the laser cavity, which was attributed to the large third-order nonlinear coefficient of ITO film. For these reasons, the multi-wavelength pulses could be achieved in our fiber laser naturally [57].

Conclusions
To sum up, the film-type ITO SA was first experimentally demonstrated using the rf magnetron sputtering technique. The film-type ITO SA fabricated using this method exhibits an excellent thermal stability, low insertion loss, large scale, and prominent continuity. Using such SA, the single-, dual-, and triple-wavelength pulses were achieved in an EDFL. Our study fully indicates this filmtype ITO SA fabricated by rf magnetron sputtering exhibits excellent nonlinear absorption characteristics, which can promote the use of ITO film in the field of ultrafast photonics.  In the experiments conducted in this study, it is noteworthy that the multi-wavelength mode-locked operations were obtained without comb filter in our fiber laser. The polarization-dependent loss attributed to the PCs and birefringence of the SMF can cause the comb filtering effect. Furthermore, there was a large nonlinearity in the laser cavity, which was attributed to the large third-order nonlinear coefficient of ITO film. For these reasons, the multi-wavelength pulses could be achieved in our fiber laser naturally [57].

Conclusions
To sum up, the film-type ITO SA was first experimentally demonstrated using the rf magnetron sputtering technique. The film-type ITO SA fabricated using this method exhibits an excellent thermal stability, low insertion loss, large scale, and prominent continuity. Using such SA, the single-, dual-, and triple-wavelength pulses were achieved in an EDFL. Our study fully indicates this film-type ITO SA fabricated by rf magnetron sputtering exhibits excellent nonlinear absorption characteristics, which can promote the use of ITO film in the field of ultrafast photonics.