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Abstract
A passively Q-switched mode-locked ytterbium-doped fiber laser (QMYDFL) at 1084.4 nm using ferroferric-oxide (Fe3O4) nanoparticles (FONPs) as the saturable absorber (SA) is reported. The FONPs SA exhibits a large nonlinear saturable absorption property with the modulation depth of 6.6% at the laser wavelength of 1µm band. Stable passively Q-switched pulse train with the repetition rate increasing from 5.43 kHz to 59 kHz, the corresponding pulse duration reducing from 18 µs to 2.68 µs are achieved when the input pump power increases from 100 mW to 330 mW. The maximum single pulse energy can reach 181.6 nJ. Q-switched mode-locked (QM) phenomenon is also observed by adjusting the state of the polarization controller (PC). When the input pump power is 150 mW, stable QM pulse train occurs. By continuous increasing the pump power to 320 mW, the QM pulse trains with the repetition rate increasing from 17.1 kHz to 34.3 kHz, mode-locked pulse repetition rate of 11.1 MHz and pulse duration of 880 ps are achieved. The observed phenomenon is interpreted as a mutual interaction of dispersion, non-linear effect and insertion loss. This work provides a new mechanism for fabricating cheap QMYDFL with FONPs SA.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Q-switched mode-locked fiber lasers (QML) are widely used for various applications such as optical communication, optical sensing, bio surgery and material processing owing to the ability to generate high-energy pulses with a submicrosecond temporal width [1–3]. The temporal output under QML exhibits a giant Q-switched envelope and two characteristic repetition frequencies (RF) shown in their frequency spectrum [4]. Recently, a great deal of research has been done on QML. One of the commonly used techniques is the incorporation of a saturable absorber (SA) into the cavity. Various SAs have been proposed to realize passive QML, including the use of semiconductor saturable absorber mirror (SESAM) [5,6] and nanomaterials with saturable absorption, such as short carbon nanotubes [7–9], graphene [10,11], topological insulators (TIs) [12–14], transition metal dichalcogenides (TMDs) [15–19] and black phosphorus (BP) [20–22]. Compared with SESAM, the nanomaterials mentioned above have broader absorption band from visible to mid-IR and faster electron relaxation time down to ∼100 fs [23]. Further, another material group called the transition metal oxides (TMOs) that includes TiO2, ZnO, Cuo, ITO, Al2O3 and Fe3O4 have been recently reported that they possesse nonlinear saturable absorption properties [24–30]. Among the TMOs, Fe3O4 is an attractive material because it has the features of high third-order non-linearity susceptibility χ(3) of 4.0 × 10−10 esu and a recovery time of 18 ∼30 ps [31,32]. Several groups have demonstrated that Fe3O4 nanoparticles (FONPs) display a semi-conductive property which has a bandgap of 0.3 eV, and the bandgap can be tuned by controlling their sizes [33]. Because nanoparticles possess a large third-order optical nonlinearity, semi-conductive property and size-dependent bandgap, which satisfy the saturable absorption condition, FONPs can be used as a Q-switcher in lasers [34]. Recently, Bai et al. utilized the FONPs as a SA in an erbium-doped fiber laser (EDFL) to achieve Q-switching operation [34]. Single wavelength and multiwavelength Q-switched fiber laser using FONPs is produced by Yushan Chen, Jinde Yin et al. [35]. Q-switched fiber laser with the pulse repetition rate increasing from 8.5 kHz to 28 kHz and the pulse duration decreasing from 23.5 µs to 6 µs by varying the pump power from 25 mW to 150 mW based on saturable absorption of FONPs is demonstrated by Dong Mao, Xiaoqi Cui et al. [36]. The potential of FONPs as a base material for SAs can also operate in the 2 µm and 3 µm wavelength regions [37,38]. Passively Q-switched Nd:YVO4 laser at 1 µm wavelength regime using the FONPs SA is reported by Xi Wang, Yonggang Wang, et al. [39]. These successful applications verify the FONPs SA’s effectiveness for passively Q-switched fiber lasers. On the base of these studies, it enlightens us to use it in QML.
In this paper, a passively Q-switched mode-locked ytterbium-doped fiber laser (QMYDFL) at 1084.4 nm using the FONPs as the SA is proposed in a ring cavity for first time. A sandwich-structured SA is fabricated by two fiber connectors, one of which is deposited a thin film of Fe3O4 magnetic fluid on the surface. A simple setup is demonstrated with the large nonlinear saturable absorption property of FONPs in combination with nonlinear polarization rotation (NPR) effect. By adjusting polarization controller (PC), Q-switched pulse train and Q-switched mode-locked (QM) pulse train could be achieved.
2. Preparation and characterization of FONPs SA
The FONPs used in our work are derived from a commercial water-based magnetic fluid (MF: Ferrotec EMG 705), which is the stable black-brown colloidal suspensions of Fe3O4 magnetic particles shown in Fig. 1(a). The surfactant used in this MF is anionic, which prevents FONP agglomeration due to van der Waals attraction. The average diameter of FONPs in MF solution is about 10 nm. The size of FONPs is very small, and their thermal energy is comparable to their gravitational potential energy. So the magnetic particles can make irregular Brownian motion and disperse uniformly, steadily and randomly in the MF system without an external magnetic field. The MF has good uniformity and high response to magnetic field. Water-based MF is in a stable colloidal state. When exposed to air for about 10 minutes at room temperature, the MF will change from stable colloidal state to a solid-state thin film. This is due to the evaporation of unsaturated-fatty-acid-based anionic surfactant and the water carrier liquid. Owing to the superior properties of MF, it is feasible to fabricate an FONP-based SA.
Fig. 1. Characterizations of Fe3O4 nanoparticle dispersion: (a) Photograph of uniform FONPs suspension. (b) SEM image of the FONPs film. (c) XRD patterns of FONPs. (d) The transmission spectrum of the FONPs
A sandwich structure of SA is prepared by a simple procedure. First, a clean optical fiber ferrule end is carefully immersed in a small amount of MF (about 0.02 ml). Because of the surface tension of the aqueous liquid, the MF is transferred to the fiber ferrule end. Next, the fiber ferrule and MF are exposed to air in a fixed vertical state for about 30 minutes to dry out at room temperature. Finally, the FONP-film-based sandwich structure of SA is obtained on the fiber ferrule and is connected to the other fiber connector with flange. The morphology and size of FONPs have been observed by the scanning electron microscopy (SEM: Quanta FEG250) seen as Fig. 1(b). It is observed that the FONPs have near spherical shape with an average diameter about 25 nm and the diameters of FONPs are uniform. But the average diameter is larger than 10 nm. This is attributed to the anionic surfactant coating film and random agglomeration of FONPs. The smaller-sized FONPs with weaker photothermal effects can improve the performances of pulsed fiber lasers [40,41]. An X-ray diffractometer (XRD: UItima IV) is used to study the crystalline structure. The XRD pattern of FONPs is shown in Fig. 1(c), which can be matched to the standard and pure spinel structure of Fe3O4 [42]. It is noted that there is a series of characteristic peaks appeared in the points of 220, 311, 400, 422, 511 and 440. The transmittance of the FONPs are measured from 900 nm to 1200 nm by using an optical spectrometer (PerkinElmer Lambda 950) shown in Fig. 1(d). It can be seen that the absorption of the film is 32.8% at 1084 nm. This linear loss may be reduced effectively by further optimizing the thickness and uniformity of the FONP film or using an FONP-coated tapered or polished fiber [43].
Because the FONPs solution SA is a key element for the stable pulse generation, the nonlinear optical saturable absorption property is investigated by using balanced twin-detector measurement system. The saturable absorption data are recorded and then fitted by the formula (1):
where $T(I )$ is the transmission, $\Delta T$ is the modulation depth, $I$ is the input intensity, ${I_{sat}}$ is the saturation power intensity, and ${T_{ns}}$ is the nonsaturable loss. According to the best fit, the nonlinear absorption characteristic is illustrated in Fig. 2. As can be seen that the modulation depth $\Delta T$, the saturable intensity ${I_{sat}}$ and nonsaturable loss ${T_{ns}}$ are 6.6%, 57.56 MW/cm2 and 17.3%, respectively.3. Experimental setup
The configuration of the proposed passively QMYDFL is shown schematically in Fig. 3. The ring-cavity consists of a 980/1064 nm WDM coupler, a segment of 5 m ytterbium-doped fiber (YDF: Nufern LMA-YDF-10/130 VIII), an optical isolation (ISO) at 1064 nm, one PC and one 90/10 optical coupler (OC). The prepared FONPs SA is inserted between the OC and the PC. The YDF with an absorption coefficient of 4dB m−1 at 975 nm acts as the gain medium and is optically pumped by the 980/1064 nm WDM coupler. A 975 nm laser diode with the maximum output power of 1 W is used as a pump source. The ISO assures the unidirectional operation of the fiber ring laser. The PC is used to rotate the polarization state and the cavity birefringence. The 10/90 OC is used to extract 10% of the lasing signal from the cavity to analyze the characteristics of the output pulse train. The total cavity length is about 18.5 m. The temporal and spectral characteristics of the QMYDFL output are recorded by Yokogawa AQ6370C optical spectrum analyzer (OSA) with 0.02 nm resolution and a 2.5 GHz battery biased InGaAs PIN photodiode combined with a 1 GHz digital phosphor oscilloscope (Agilent Technologies DSO6102A).
4. Results and discussion
The self-starting Q-switched pulse trains begin to appear at 100 mW of pump power. The corresponding average output power is 0.38 mW. The pulse duration, repetition rate and single pulse energy are measured to be 18 µs, 5.43 kHz and 93 nJ, respectively. The laser pulse trains at the pump power of 103 mW, 111 mW, 158 mW, 216 mW, 250 mW, 291 mW and 310 mW are shown as Fig. 4(a) respectively. As can be seen that the pulse trains have uniform intensity, which illustrate the stability of the pulse operation state. And both the repetition rate and pulse duration are pump power dependent, which is a typical feature of a Q-switched laser. For a typical operation state at pump of 150 mW, a stable pulse train is obtained with pulse duration of 4 µs and repetition rate of 16.1 kHz shown in Fig. 4(b). The average output power is 3.1 mW and the corresponding single pulse energy is 181.57 nJ. A typical Q-switched optical spectrum measured at the pump power of 150 mW is shown in Fig. 4(c). The central wavelength and 3 dB bandwidth are 1084.4 nm and 0.44 nm, respectively. The dependences of average output power and single pulse energy on the pump power are shown as Fig. 4(d). The generated pulse train produce a maximum output power and a maximum pulse energy of 10 mW and 180 nJ, respectively. It can be seen that the output power increases almost monotonically with the pump power, while the pulse energy increases in the beginning and then decreases when the pump power is higher than 150 mW. Such an abnormal phenomenon can be attributed to the degeneration of SA that caused by laser-induced heat accumulation. The repetition rate and the corresponding pulse duration versus the pump power are shown in Fig. 4(e), where the repetition rate increases from 5.43 to 59 kHz. The pulse duration, in contrast, decreases from 18 to 2.68 µs. This phenomenon is explained as the nonlinear dynamics of the gain medium and the SA. With the pump power increasing, more gain is supplied to saturate the SA and the threshold energy stored in the gain medium reaches faster to generate a pulse. As a result, the pulse is generated more quickly, thus decreasing the pulse duration while increasing the repetition rate.
Fig. 4. Laser performance of FONPs-based passively Q-switched YDFL. (a) Typical Q-Switched pulse trains at different pump power. (b) Single pulse envelope at pump power of 150 mW. (c) Output spectrum. (d) Output power and pulse energy versus pump power. (e) Repetition rate and pulse duration versus pump power.
When the pump power is 150 mW, the system also showed other different dynamics. By carefully adjusting the state of the PC, continuous-wave (CW) laser operation, Q-switched laser operation and QML operation are overserved from the the laser outputs seen as Fig. 5(a). The central wavelength slightly drifts because the cavity length will change slightly when the state of PC is changed. In different operations, the intensity of the wavelength and the number of the wavelength are slightly different because of the mode competition. It can be seen the spectrum of the QML shows multi-wavelength operation of the laser, where the triple-wavelength oscillations are achieved at 1079.1 nm, 1082.9 nm and 1085.1 nm, the 3dB bandwidth of the 1085.1 nm is 0.8 nm. The Q-switched pulses of the QMYDFL are recorded by the digital phosphor oscilloscope seen as Fig. 5(b). It can be seen the Q-switched pulse involved some small modulations, that is to say QML pulse trains are observed. In the YDFL at 1µm band, the fiber is usually in the positive dispersion range. In order to realize the mode locking, the whole laser system is very dependent on the SA and the filter effect in the cavity. The QM pulse trains at the pump power of 150 mW, 182 mW, 203 mW, 235 mW, 264 mW and 280 mW are shown as Fig. 5(b) respectively. As can be seen that the QM pulse sequence is stable. And the repetition rate of Q-switched pulse envelope is pump power dependent, but the mode locked pulse repetition rate is independent of the pump power, which is a typical feature of QML. For a typical operation state at pump of 203 mW, the Q-switched envelope repetition rate is 18.1 kHz. The typical QM pulse envelop is seen as Fig. 5(c). And the mode-locked pulse repetition rate is 11.1 MHz. The oscilloscope traces of mode-locked pulse trains are shown in Fig. 5(d). The mode-locked pulse duration is achieved seen as Fig. 5(e). The real rise time ($tr$) of mode-locked pulse can be calculated by using the formula (2):
where ${t_{m}}$ is the measured rise time of mode locked pulses, ${t_{P}}$ is the rise time of the photodiode, ${t_{0}}$ is the rise time of the oscilloscope. The estimated single mode-locked pulse duration is about 880 ps. According to the relationship between the cavity length L and the frequency f in the mode-locked principle, $f = {\raise0.7ex\hbox{$c$} \!\mathord{\left/ {\vphantom {c {nL}}} \right.}\!\lower0.7ex\hbox{${nL}$}}$, where c represents the speed of light in vacuum, n is the refractive index of the transmission medium, when the fiber length is 18.8 m, the corresponding mode-locked repetition rate is 11.1 MHz, that is to say, the mode-locked pulse interval is 90 ns. It can be seen from Fig. 5(d) that the mode-locked pulse fully conforms to the relationship between cavity length and frequency. So the observed phenomenon is the QM phenomenon.Fig. 5. Laser performance of FONPs-based passively QMYDFL. (a) Output spectra at different operation states (b) Typical QM pulse trains at different pump power. (c) Oscilloscope traces of a typical QM pulse envelop at pump power of 200 mW. (d) Oscilloscope traces of mode locked pulses trains. (e) Single pulse envelope.
5. Conclusion
A passively QMYDFL at 1084.4 nm employing FONPs as the SA is demonstrated. A sandwich-structured SA is fabricated by two fiber connectors, one of which is deposited a thin film of Fe3O4 magnetic fluid on the surface. The laser architectures are constructed and their performances are evaluated. By continuous increasing the pump power from 100 mW to 330 mW, stable passively Q-switched pulse train with the repetition rate increasing from 5.43 kHz to 59 kHz and the corresponding pulse duration reducing from 18 µs to 2.68 µs are achieved. The maximum single pulse energy can reach 181.6 nJ. QM phenomenon is also observed by adjusting the state of the PC. When the input pump power is 150 mW, stable QM pulse train occurs. By continuous increasing the pump power to 320 mW, QM pulse train with the repetition rate from 17.5 to 35 kHz, mode-locked pulse repetition rate of 11.1 MHz and pulse duration of 880 ps are achieved. Due to the large nonlinear saturable absorption property, the potential of FONPs SA is valuable for QML in high power operation, which is desirable pulse source from basic research to industrial applications.
Funding
National Key Scientific Instrument and Equipment Development Projects of China (61627814); State Key Laboratory of Advanced Optical Communication Systems and Networks.; Undergraduate Innovative Test Program of China (BEIJ2019110001, URTP2019110008, URTP2019110009).
Acknowledgements
We acknowledge financial support from the National Key Scientific Instrument and Equipment Development Project (Grant No: 61627814), Undergraduate Innovative Test Program of China (URTP2019110009, URTP2019110008, BEIJ2019110001) and State Key Laboratory of Advanced Optical Communication Systems and Networks.
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