Yttrium oxide as a Q-switcher for the near-infrared erbium-doped fiber laser

1 Introduction

Passively Q-switched fiber lasers (QSFL), which are generated by Q-factor modulation or intracavity loss regulation, have attracted much attention because of their intrinsic advantages of high energy, alignment-free structure, compactness, and high stability [1], [2], [3], [4]. Up to now, QSFLs have been widely applied in medicine, industrial material processing, fiber-optical sensing, and optical communication [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]; but they can also be used as an ideal platform for investigating the dynamic evolution of solitons and saturated absorption of nanomaterials [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33].

In recent years, passively QSFLs based on saturable absorbers (SAs) have received much attention. Semiconductor saturable absorber mirrors (SESAMs) are considered to be the hot topic in commercial applications. However, the inevitable features of complex manufacturing process, narrow bandwidth, and high cost make it hard for them to meet future commercial application requirements [34], [35]. Graphene, which has emerged as required by time, has been attracting growing attention in recent years. The characteristics of ultra-broadband absorption and ultrafast electron dynamics make it shine brilliantly in the field of optoelectronics [36], [37]. The tremendous success of graphene has also led to the exploration of more potential materials [38], [39], [40], [41], [42]. Transition-metal dichalcogenides (TMDs) possess unique photoelectric properties that vary with thickness. They not only exhibit ultra-wideband absorption characteristics due to defective states but also have outstanding performance in realizing ultrafast lasers [43], [44], [45]. Similarly, topological insulators (TIs) and black phosphorous (BP) have recently got breakthroughs in applications of photonics and photoelectronics in the near-infrared band [46], [47], [48], [49]. In addition, more new materials with excellent properties are beginning to emerge [50], [51], [52].

Yttrium oxide (Y₂O₃) performs well in terms of hardness, melting point, and chemical stability. Therefore, it is often used in metal-reinforced composites, microelectronics, waveguide lasers, and high-temperature protective coatings [53], [54], [55]. In microelectronics, Y₂O₃ is considered as a potential gate dielectric material because of its high dielectric constant and good compatibility with silicon [56]. Moreover, because of the excellent thermo-optical properties of Y₂O₃, it can be easily doped with a variety of rare earth ions, thus helping to produce high-power waveguide lasers [57], [58]. However, few studies have been done on the nonlinear optical applications of Y₂O₃ as SAs in fiber lasers so far.

In order to stimulate the application of Y₂O₃ in more fields, we attempted to explore its optical nonlinearity. Combined with the common magnetron sputtering method, the tapered Y₂O₃ SA was prepared. By coupling the Y₂O₃ SA into the cavity, a QSFL delivering an average output power of 26 mW at 1530 nm with a pulse duration of 592.7 ns was obtained. Our experiments not only prove the optical nonlinearity of Y₂O₃ but also provide the possibility for the further development of optical modulation and optoelectronic devices.

2 Preparation and characterization of Y₂O₃ SA

To enhance the nonlinearity and reduce thermal damage of the Y₂O₃ SA, the tapered fiber structure was selected. Meanwhile, to ensure convenience and efficiency of production, we chose the appropriate production method, namely magnetron sputtering deposition (MSD). The clean, tapered optical fiber, which was prepared in advance, had a waist diameter of 14 μm and effective fused zone length of 0.8 cm; the large effective length of the fused zone and the small waist diameter of the tapered fiber help in enhancing the nonlinearity of the Y₂O₃ SA. The specific production process is as follows: First, the commercially purchased Y₂O₃ target with 99.99% purity was tapered and fixed in a vacuum chamber. A vacuum pump was used to bring the degree of vacuum to 10×10⁻³ Pa. Then, excited and accelerated Ar ions were made to bombard the Y₂O₃ target under the action of an electric field. Subsequently, the sputtered Y₂O₃ particles were uniformly deposited on the outer wall of the optical fiber. Meanwhile, the fiber was rotated evenly at a speed of 10 rpm to make the material dense and uniform. The flow rate of Ar was 20 sccm (standard cubic centimeter per minute) during the the sputtering process. The temperature of the preparation process was 200°C.

After the preparation of the Y₂O₃ SA, some necessary characterization and measurements were carried out. Atomic force microscopy (AFM) was used to examine the surface properties of the prepared Y₂O₃ SA (Figure 1). By detecting the thickness difference between the material coverage area and the silicon substrate in Figure 1A, the exact thickness of Y₂O₃ was estimated as 7 nm in Figure 1B. According to the definition of two-dimensional (2D) materials, the Y₂O₃ used here can be considered as two dimensional [59], [60]. Meanwhile, the surface morphology of Y₂O₃ is shown in Figure 1C, which reveals the compactness and uniformity of the material surface.

Figure 1:

The AFM of Y₂O₃.

(A) Material boundary, (B) thickness, and (C) surface morphology of Y₂O₃.

X-ray photoelectron spectroscopy (XPS) is able to effectively determine the composition and properties of materials. As shown in Figure 2A, the prominent O-Y, Y-O-Si, and O-Si peaks are located at 529.8, 531.9, and 532.9 eV, respectively. The Y-O-Si peak that is observed in the film may be the result of diffusion of the substrate silicon [61]. As shown in Figure 2B, the prominent Y 3d_5/2 and Y 3d_3/2 peaks are located at 157 and 159 eV, respectively. The XPS spectrum of Y₂O₃ is highly consistent with those obtained in previous studies [61], [62]. The agreement in both composition and binding energy proves the existence of Y₂O_3.

Figure 2:

The XPS of Y₂O₃ film.

(A) O 1s spectra of Y₂O₃ films. (B) Y 3d spectra of Y₂O₃ films.

The absorption spectrum of Y₂O₃ is shown in Figure 3A, which indicates that the as-prepared Y₂O₃ has ultrawide absorption characteristics. The absorptivity of the fabricated Y₂O₃ was measured as 38.346% at 1550 nm. In the investigation of the optical nonlinearity of Y₂O₃, the twin-detector measurement technique was used. The light transmissions of Y₂O₃ at different powers were recorded separately, which are shown in the figure as blue points. The pump source during the measurement was operated at 1550 nm, and the corresponding pulse duration and repetition rate were 700 fs and 120 MHz, respectively. The results were fitted by

Figure 3:

Linear and nonlinear absorption characterization of Y₂O₃.

(A) Absorption spectrum of Y₂O₃ films. (B) Nonlinear absorption characteristics of the Y₂O₃ SA.

T=1−(αs1+I/Isat+αns).

The results of curve-fitting in Figure 3B show that the Y₂O₃ SA has a modulation depth (α_s) of 46.43%, nonsaturable loss (α_ns) of 26.75%, and saturable intensity (I_sat) of 1.58 MW∕cm². The large modulation depth may come from two aspects: the strong nonlinearity of the Y₂O₃ material itself, and the small waist diameter and large effective length of the fused zone of the tapered fiber. The performance comparison of the Y₂O₃ SA and other nanomaterial-based SAs is shown in Table 1. The relatively large modulation depth of the Y₂O₃ SA is beneficial to the generation of ultrashort pulses. Meanwhile, the small saturation intensity of the Y₂O₃ SA is conducive to a low start threshold of the Q-switched laser. The insertion loss (IL) of the Y₂O₃ SA is 1.4 dB, which is the average value.

3 Experiment

Considering the superiority of the fiber laser in terms of alignment-free structure, compactness, and high stability, it was chosen as the platform for the nonlinearity verification of Y₂O₃. Figure 4 is the device diagram of a commonly used ring-cavity fiber laser. The length of the erbium-doped fiber (EDF) used as the gain medium is 60 cm, and the total length of the cavity is 2.1 m. The source in the cavity is a commercial laser pump operating at 980 nm. As an important device for coupling the light source into the cavity, the wavelength division multiplexer (WDM, 980/1550nm) can combine optical signals of different wavelengths into one bundle. The isolator (ISO) ensures the unidirectional transmission of light, thereby guaranteeing the normal operation of the laser and avoiding unnecessary device damage. The intracavity birefringence and polarization state are adjusted by a polarization controller (PC), thus optimizing the operating state for a stable pulse output. With an 80:20 optical coupler (OC), 20% of the intracavity signals are exported for real-time monitoring. The Y₂O₃ SA is placed between the WDM and ISO. Optical devices such as an oscilloscope (Tektronix DPO 3054) and an optical spectrum analyzer (Yokogawa AQ 6370C) outside the cavity are used to monitor and measure the real-time dynamics in the cavity.

Figure 4:

Experimental installation diagram of the proposed fiber laser.

4 Results and discussion

After inserting the Y₂O₃ SA into the cavity, stable Q-switched pulses appeared at the pump power of 148 mW. The pulse sequences at different pump powers are shown in Figure 5A. In Figure 5B, the spectrum of the realized Q-switched operation has a central wavelength of 1530 nm with a bandwidth of 1.5 nm. It is worth mentioning that there is no significant change in the spectral shape recorded at different time intervals. Meanwhile, there is no remaining pump power from the output. The monopulse envelope of the output pulse at 630 mW is shown in Figure 5C, whose typical symmetrical Gaussian shape shows a pulse duration of 592.7 ns. The radio frequency (RF) spectrum measured is shown in Figure 5D. At the resolution bandwidth of 10 Hz, the Q-switched system exhibits a signal-to-noise ratio (SNR) of 65 dB. Meanwhile, the frequency-doubled signal decreased evenly over a wide range of frequencies, which further proved the stability of the system.

Figure 5:

Performance of QSFL based on Y₂O₃ SA.

(A) Pulse sequence at different pump powers. (B) Spectra for different time periods. (C) Pulse duration of 592.7 ns. (D) RF spectrum.

With the increase of input power, the repetition rate of output pulse increases almost uniformly in the range 112–217 kHz, as shown in Figure 6A. The pulse duration decreases rapidly in the initial stage of power growth and tends to be stable in the later stage, which is related to the pump-induced gain compression effect as confirmed in Ref. [68]. The stability of the later pulse duration indicates that the absorber tends toward saturation. The adjustable range of pulse duration is 2259–592.7 ns when input power changes from 148 to 630 mW. The repetition rate of the Q-switched pulse increases with the increase of the pump power as reported in Ref. [69]. From Figure 6B, for a pump power of 630 mW, the average output power and pulse energy are 26 mW and 120 nJ, respectively. It is worth mentioning that at the maximum pump power, the growth trend of the average output power is still upward, which indicates that the stable working condition can be maintained even at high power. The maximum damage threshold of the Y₂O₃ SA is ~68 mJ/cm². In the experiment, mode-locking was not obtained because the nonlinearity and anomalous dispersion did not reach equilibrium in this case. Attempts to apply the Y₂O₃ SA in mode-locked lasers will continue in future work.

Figure 6:

Characteristics of proposed QSFL variation under different power.

(A) Pulse duration and repetition rate as functions of input power. (B) Average output power and pulse energy as functions of input power.

The comparison between the proposed QSFL and previous lasers is shown in Table 2. The pulse duration of 592.7 ns is proved to be competitive among SA-based QSFLs. Moreover, the high melting point and good chemical stability of the Y₂O₃ enable the Q-switched oscillator in maintaining stable, high power. Therefore, the Y₂O₃ SA may make inroads into the potential market of optical modulation and optoelectronic devices.

5 Conclusion

An erbium-doped QSFL using Y₂O₃ as the Q-switched device to deliver nanosecond pulses has been demonstrated in this article. The generated stable Q-switched pulses have a controlled repetition rate of 112–217 kHz, pulse duration of 593 ns, output power of 26 mW, and pulse energy of 120 nJ. The pulse duration of 593 ns is comparable with that of other SA-based QSFLs. Moreover, the high melting point and good chemical stability Y₂O₃ result in improved stability of the laser even at high power. Our experiments have shown that the Y₂O₃ SA not only has strong nonlinearity and advantages in achieving ultrashort pulse duration but also has worked steadily in high-power operation. Therefore, the Y₂O₃ SA may make inroads into the potential market of optical modulation and optoelectronic devices.