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Abstract
Chromium oxide (Cr2O3) is a promising material used in the applications such as photoelectrochemical devices, photocatalysis, magnetic random access memory, and gas sensors. But, its nonlinear optical characteristics and applications in ultrafast optics have not been studied yet. This study prepares a microfiber decorated with a Cr2O3 film via magnetron sputtering deposition and examines its nonlinear optical characteristics. The modulation depth and saturation intensity of this device are determined as 12.52% and 0.0176 MW/cm2. Meanwhile, the Cr2O3-microfiber is applied as a saturable absorber in an Er-doped fiber laser, and stable Q-switching and mode-locking laser pulses are successfully generated. In the Q-switched working state, the highest output power and shortest pulse width are measured as 12.8 mW and 1.385 µs, respectively. The pulse duration of this mode-locked fiber laser is as short as 334 fs, and its signal-to-noise ratio is 65 dB. As far as we know, this is the first illustration of using Cr2O3 in ultrafast photonics. The results confirm that Cr2O3 is a promising saturable absorber material and significantly extend the scope of saturable absorber materials for innovative fiber laser technologies.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Nonlinear optical materials play an increasingly crucial role nonlinear photonics [1,2]. In fact, they have enabled many currently used photonic devices such as photodetectors, pulsed lasers, optical modulators, and optical switches, highlighting the unparalleled advantages of optical techniques over their electronic counterparts [3,4]. Saturable absorbers (SAs) are typical applications of nonlinear optical material devices in ultrafast pulse lasers, including biology, optical communication, machining and laser processing. The saturable absorption of nonlinear optical materials is a nonlinear optical characteristic relating to the intensity of light, mainly expounded using the Pauli exclusion theory [5,6]. The current SAs include commercial SAs such as semiconductor saturable absorber mirrors (SESAMs) and low-dimensional material- based SAs [7,8]. Although SESAMs have been investigated for a long time, their development and application are restricted owing to the insignificance of controllable absorption that as an important parameter for the assessment of SA performance. Further, low-dimensional materials based SAs, which widen the regulatory range of nonlinear optical absorption, have gradually supplanted SESAMs because of their remarkable characteristics such as high third-order nonlinear optical susceptibility, controllable bandgap, and rapid carrier mobility [9,10]. In the past decade, ranging from graphene, topological insulators, and transition metal dichalcogenides, to single-element materials, low-dimensional materials with excellent performance in wide wavelength bands, ultrafast carrier recovery times, large modulation abilities, and good stabilities, have been developed and successfully utilized as SAs in passively Q-switched and mode-locked lasers [11–15]. However, the photon absorption efficiency of monolayer graphene is deficient, significantly limiting the further enhancement of SAs’ performance [16]. Single-element materials are widely studied SAs whose thickness-dependent bandgap properties are suitable for wide-band optical applications. However, the drawbacks of easy oxidation and instabilities limits their practical applications [17]. Transition metal dichalcogenides possess the merits of high absorption efficiency and strong light-matter interactions. But, their recovery time of excited carrier is slow, and carrier mobility is small [18,19]. Consequently, it is essential to explore a new type of SA with outstanding performance. In recent years, oxide saturable absorbers showed great potential in pulsed lasers for they possess commonly relatively high damage threshold due to low chemical activity compared to their elemental form [20]. Chromium oxide (Cr2O3), as p-type semiconductors, exhibiting good conductivity and optical transparency, is the most stable among various chromium oxide solid phases, which has garnered increasing attention in view of its important advantages of low humidity dependency, high thermal stability and high chemical stability [21,22]. It has been found that the Cr2O3 possesses good optical quality and its bandgap can be widely tuned by changing the oxygen content [23]. In addition, theoretical studies have demonstrated that Cr2O3 has excellent photoelectric properties [24,25]. Despite being a promising material, no previous study has evaluated Cr2O3 materials for ultrafast photonic applications in fiber lasers.
In this paper, we report a passively mode-locked and Q-switched Er-doped fiber (EDF) laser with Cr2O3-microfiber as the SA. The SA is prepared by decorating a Cr2O3 film on the tapered zone of a microfiber via magnetron sputtering deposition. The nonlinear saturable absorption features of the proposed SA are determined using a balanced two-detector measurement system. The modulation depth and saturation intensity of the SA are 12.52% and 0.0176 MW/cm2. The Cr2O3-microfiber SA is applied to the EDF laser, and Q-switching/mode-locking operation is realized. The repetition frequency of the Q-switched fiber laser is tuned in the range of 161.2-208.9 kHz, possessing the narrowest pulse width of 1.385 µs and the highest output power of 12.8 mW. In the mode-locked working state, ultrafast laser pulses are formed with a pulse width of 334 fs and a repetition rate of 34.48 MHz. As far as we know, this is the first application of Cr2O3 in ultrafast photonics.
2. Experiments
This study used the magnetron sputtering deposition method to prepare the Cr2O3 film as it shows the strong points of easy operation and low cost [26]. The Cr2O3 raw material with a purity of 99.99% was selected as the target material. Microfibers made of SMF-28e fibers owing a waist diameter of 12 µm and fused zone of 1 cm act as the substrates. The evanescent-field interaction method can enhance the light modulation ability of SA by extending the interaction length between light and materials. Therefore, the microfiber structure is often used, resulting in strong evanescent-material interaction and improvement of the SA damage threshold. Figure 1(a) describes the magnetron sputtering process. The sputtering chamber was pulled to a high vacuum degree. Then, Ar gas was ionized by a strong electric field and bombarded the target. Finally, the inspired Cr2O3 plasma plume was deposited on the microfibers. After the deposition process, the Cr2O3 film wrapped around the evanescent field region of the microfibers. The films obtained by the magnetron sputtering deposition method are expected to exhibit better optical and electrical characteristics owing to a remarkably reduced number of grain boundaries. The chemical composition of Cr2O3 were tested through X-ray photoelectron spectroscopy (XPS). Figure 1(b) gives the full XPS spectra of Cr2O3; we can observe five peaks binding energies of 285, 400, 530, 577, and 586 eV corresponding to C 1s, N 1s, O 1s, Cr 2p3/2, and Cr 2p1/2, separately. In the O 1s XPS spectra of Cr2O3 (Fig. 1(c)), an apparent peak is located at 530 eV, originating from the O-Cr band. Figure 1(d) shows the spectrum of Cr 2p, where two peaks are observed at binding energies of 586 eV and 577 eV, corresponding to the Cr 2p1/2 and Cr 2p3/2 states, separately. The Raman spectrum of Cr2O3, as shown in Fig. 2(e), shows four typical peaks of Cr2O3 at approximately 305, 345, 545, and 604 cm−1. The peaks at 305, 345, and 604 cm−1 correspond to the Eg modes and the peak at 545 cm−1 corresponds to the A1g mode. All the material characteristic test results are consistent with those of the reported studies [27,28].
Fig. 1. (a) Schematic of magnetron sputtering; (b) the XPS spectra of Cr2O3; (c) The O-1s spectra of Cr2O3; (d)The Cr-2p spectra of Cr2O3; (e) The Raman spectrum of Cr2O3.
Fig. 2. (a) Top views of bulk Cr2O3; (b) atomic structure of bulk Cr2O3; (c) The setup of balanced two-detector measurement; (d) nonlinear transmittance of the Cr2O3-microfiber SA.
Figures 2(a) and 2(b) show the atomic structure of Cr2O3, where the blue balls represent Cr atoms and the red balls represent O atoms. The lattice parameter a(b) = 4.955 Å, and c = 13.6 Å. Standard Cr2O3 exhibits a hexagonal corundum structure. In the unit cell, Cr atoms occupy two-thirds of the interstitial octahedral sites, whereas O atoms constitute a hexagonal close-packed structure. The nonlinear optical absorption properties of Cr2O3-microfbier SA were studied via a balanced two-detector system, as is shown in Fig. 2(c). A home-made ultrafast laser acts as the optical source with a pulse width, central wavelength, and pulse repetition rate of 700 fs, 1550 nm, and 120 MHz, respectively. An optical attenuator is used to regulate the incident light intensity and realize various transmittances. The optical source power was divided into two equal beams using a 50:50 optical coupler. One beam was set as the reference light, and the other was used as the probe light to record the light intensity passing through the Cr2O3-microfbier SA. The optical transmission of Cr2O3-microfbier SA under different incident powers was then recorded using a power meter with a dual-channel. The experimental data are provided in Fig. 2(d). The blue balls represent the experimental data, and the red line represents the fitted curve, which is fitted using the widely-used two-level saturable absorber model, as demonstrated in formula (1):
where T(I) is the transmittance, and I is the incident optical intensity. The saturation intensity (Isat), modulation depth (αs), and non-saturation loss (αns) were estimated to be approximately 12.52%, 0.0176 MW/cm2, and 67.48%, respectively. The relatively large modulation depth of 12.52% is conducive to the generation of ultra-fast mode-locked pulses. The low saturation intensity value enables easier Q-switching or mode-locking operation, implying that the proposed SA is more universal for laser modulation. In this work, the Cr2O3 film shows a remarkable saturable absorption property at approximately 1550 nm, corresponding to a photon energy of 0.8 eV, which is lower than the bandgap energy of bulk Cr2O3 (3.3 eV) [29,30]. In general, light absorptivity depends on the structural defects in the materials, and defects are easily introduced during the growth process. It should be noted that major defects of O vacancies exist in the deposited Cr2O3 films, and the existence of O vacancies can induce a reduction in the bandgap energy [31]. Therefore, the saturable absorption of the Cr2O3 film at approximately 1550 nm could be demonstrated owing to the contribution of defects of O vacancies introduced during the Cr2O3 film preparation on the microfibers.An EDF laser was built with a Cr2O3-microfiber as the SA. Figure 3 displays the structural diagram. The pump source was a 980 nm laser diode (LD). The pump light was coupled into EDF cavity through a 980/1550 nm wavelength division multiplexer (WDM). A 40-cm long EDF (Liekki 110-4/125) was employed as the gain medium. A 10/90 optical coupler (OC) was utilized to extract 10% of the light from the fiber cavity for pulse characterization using a digital oscilloscope and an optical spectrum analyzer. However, 90% of the light returned to the fiber cavity. An isolator (ISO) with a polarization-independent feature was spliced into the ring cavity to guarantee the unidirectional operation of the laser. To realize laser pulse operation, the prepared microfiber decorated with the Cr2O3 film, used as SA, was inserted into the ring cavity. In addition, a polarization controller (PC) was added between the OC and ISO to control the intra-cavity birefringence and polarization states. The pigtails of all optical devices were SMF-28e fibers. The dispersion parameters of SMF-28e and EDF were -22 and 12 fs2, respectively. The total cavity length was 5.5 m, and the total dispersion was calculated as -0.107 ps2.
3. Results and discussion
When the pump power is enhanced to 560 mW, a Q-switching pulse sequence is clearly noticed. The Q-switched threshold of 560 mW is slightly higher, which is primarily attributed to the large splice loss in the fiber laser. The EDF (Liekki 110-4/125) has a core diameter of 4 µm, which could not match with SMF-28e fiber (diameter: 8.2 µm). Therefore, the fiber cavity loss is relatively larger and the threshold of the fiber laser is slightly higher. Stable pulse trains under different pump powers are presented in Fig. 4(a). The Q-switched state can be maintained with a pump power of 560-980 mW. Figure 4(b) displays the profile of single-pulse at a pump power of 980 mW, which exhibits a Gaussian structure. The pulse duration is measured as 1.385 µs, which is the narrowest pulse width of the Q-switched fiber laser. The measured optical spectrum, as provided in Fig. 4(c), contains two peak wavelengths at 1531.4 nm and 1552.5 nm, respectively. The dual-wavelength operation is attributed to the nonlinear optical property of microfiber decorated by Cr2O3 film and gain bandwidth limitation effect [32,33]. Figure 4(d) presents the radio-frequency (RF) spectrum of the Q-switching pulses over a span of 1800 kHz. The signal-to-noise ratio (SNR) is tested as 65 dB at a pump power of 980 mW, confirming the stable state of this Q-switching operation. Figure 4(e) illustrates the evolution of the pulse width and repetition frequency with increasing pump power. The increasing trend of the repetition frequency of the Q-switched fiber laser with the increase of pump power is a representative Q-switched feature that is clearly observed. The repetition frequency increases from 161.2 to 208.9 kHz. The pulse width exhibits a reducing trend with the increase of pump power. More specifically, the pulse duration narrows from 2.5 to 1.385 µs with the enhancing pump power from 560 to 980 mW. Figure 4(f) shows the changes in the average output power and single-pulse energy with increasing pump power. The output power shows a linear increase with the increase of pump power. The largest output power and single-pulse energy are 12.8 mW and 61.3 nJ, separately.
Fig. 4. Q-switched laser performance. (a) Pulse sequences at different pump powers; (b) single-pulse profile; (c) Q-switched spectra; (d) RF spectra; (e) the changes of repetition frequency and pulse width; (f) the changes of output power and single-pulse energy.
Under pump power of 580 mW, a mode-locked operation for ultrafast laser pulse generation is observed via rotating the PC to regulate the polarization state in the intra-cavity. Figure 5(a) exhibits the spectra of the mode-locked pulses. The center wavelength is located at 1561.9 nm, and the 3-dB bandwidth is 10 nm. Kelly sidebands are clearly seen on the spectrum shoulder, suggesting that the fiber laser is in the conventional soliton mode-locking state. Due to the periodical disturbances of gain and loss, the solitons release the excess energy as dispersive. When the phase difference between the soliton and the dispersive wave is multiples of 2π, strong interference occurs and Kelly sidebands are formed [34,35]. The Fig. 5(b) shows the trace measured in a span of 2 µs, indicating that the pulse amplitude is uniform, and confirming that the laser works in the continuous wave mode-locked mode. The RF spectrum in a span of 500 MHz is presented in Fig. 5(c), indicating that the fundamental repetition frequency of 34.48 MHz is consistent with the ring cavity repetition rate and the SNR value is as high as 65 dB. Furthermore, no spurious sideband is observed, thus demonstrating the high spectral purity of this mode-locked performance. Figure 5(d) illustrates the autocorrelation trace of mode-locked pulses, which means that the pulse duration is 334 fs when the sech2 fitting function is used. Consequently, the time-bandwidth product (TBP) of the soliton pulse can be obtained theoretically via the formula (2):
where τ means the pulse width (334 fs), c represents the constant of light velocity, Δλ equals to the 3 dB width of spectra (10 nm) and λc is the central wavelength (1561.0 nm). These obtained data indicate the TBP is 0.41, which is higher than the transform-limited value of 0.315, implying that the soliton pulses have some chirp. To evaluate the long-term stability of the mode-locking operation, we continuously monitor the evolution of optical spectra every 2-hour over 9 hours. Notably, the central spectral peak locations, spectral bandwidth, spectral intensity remained reasonably stable over the time period, indicating the long-term stability is good.Fig. 5. Mode-locked laser performance. (a) Mode-locked spectra; (b) Mode-locked pulse sequence; (c) the RF spectrum; (d) autocorrelation trace.
In this work, all used components in cavity are polarization insensitive. Hence, the mode-locking or Q-switching mechanism is dependent only on the saturable absorption of Cr2O3-based SA, and the laser working state can be transformed from the Q-switching operation to the mode-locking operation by regulating the polarization state inside the fiber cavity. There are two transverse modes in the ring fiber cavity: transverse magnetic (TM) mode and transverse electric (TE) mode. As the TM mode polarization state is vertical to the Cr2O3 film decorating surface, it shows low interaction. In contrast, the TE mode polarization state is parallel to the decorating surface, and thus the evanescent field acquires a high interaction with the materials [51,52]. The PC works by applying pressure with an adjustable clamp. The pressure on the fiber causes a birefringence within the fiber core, and it can make fiber laser operate in a proper state with enough nonlinear effect. So the PC has the function of adjusting the phase, providing optimal phase retardation and transmission losses. As for Q-switching operation, the cavity Q-factor can be modulated periodically when the gain and loss reach to the equilibrium by adjusting the PC and pump power. So the pulse train with a kilohertz repetition rate is emitted. As for mode-locking operation, the net cavity dispersion of -0.107 ps2 and the Cr2O3-microfiber SA enhances the nonlinearity. So the equilibrium of dispersion and nonlinearity is easy to reach through controlling PC. Therefore, the Q-switching operation can switch to a mode-locking operation by regulating the transverse mode of the propagating light by modifying the PC position. To further study the laser performance, the obtained laser results are compared with that of other SA-based fiber lasers, as provided in Table 1 and Table 2. The comparison presented in Table 1 confirms that the obtained pulse width is comparable to those of the other SAs. In addition, the output power and SNR have some advantages over other results. Table 2 shows that the proposed SA has an excellent modulation ability with a modulation depth of 12.52%. The 3-dB width of the optical spectrum is as wide as 10 nm, indicating that this fiber laser has the ability to generate ultrashort pulses. A pulse width of 334 fs shows better performance, surpassing the other results, as shown in Table 2. The SNR is of the same order as the reported values, illustrating the good stability of the mode-locked working state. Thus, Cr2O3, as a new SA material, shows impressive comprehensive performance and potential, and this study is expected to trigger further extensive studies on the optical properties of Cr2O3.
Table 1. Summary of Q-switched fiber lasers based on different SAs.
Table 2. Summary of mode-locked fiber lasers based on different SAs.
4. Summary
In summary, microfibers decorated with Cr2O3 film were prepared via magnetron sputtering deposition. Further, this study investigated the nonlinear saturable absorption characteristics, which showed a saturation intensity of modulation depth of 12.52% and 0.0176 MW/cm2. Based on the saturable absorption of the Cr2O3-microfiber SA, stable Q-switching and mode-locking operations are demonstrated. In Q-switched working state, stable Q-switched pulses are generated with the narrowest pulse width of 1.385 µs, largest output power of 12.8 mW, and SNR of 65 dB. In the mode-locked operation, the ultrafast EDF laser possesses a pulse duration of 334 fs and a high SNR of 65 dB. Thus, the performance and potential of Cr2O3 are impressive, and it possesses a remarkable potential for ultrafast photonic applications.
Funding
National Key Research and Development Program of China (2022YFB4601101); National Natural Science Foundation of China (11875044, 12075190, 62004162); New Star Project of Science and Technology of Shaanxi Province (2022KJXX-69); Innovation Capability Support Program of Shaanxi (2021TD-09).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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