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Abstract
We report on a passively Q-switched Nd:YVO4 laser at 1064.34 nm by using the ferroferric-oxide (Fe3O4) nanoparticles (FONPs) saturable absorber (SA). It is corroborated that the FONPs SA exhibits a large nonlinear saturable absorption property with the modulation depth of 2.49% at the laser wavelength of 1 µm. By inserting the novel SA into a V-type Nd:YVO4 laser cavity, we obtain the shortest pulse duration of 53 ns with a repetition rate of 576.4 kHz. The corresponding average output power, single pulse energy, and peak power are 104 mW, 0.18 µJ, and 3.53 W, respectively. To the best of our knowledge, it is the first time to experimentally confirm the application of FONPs in a pulsed Nd:YVO4 solid state laser. The parameters of the pulse width, average output power, and peak power are superior to those in the reported pulsed fiber lasers with FONPs SA so far.
© 2017 Optical Society of America
Corrections
20 July 2017: A minor correction was made to Fig. 2.
1. Introduction
Short pulse Q-switched solid state lasers (SSLs) with pulse duration of sub-100 ns and super-100 ps are always applied extensively to the fields of military, material processing, medical, and scientific research owing to the characteristics of high output power, large pulse energy, and simple manufacture [1–5]. The primary impetus behind the development of solid state lasers roots in the diversification of gain materials and devices. As the important nonlinear saturable absorption devices to generate short laser pulses, the kinds and amount of SAs have grown rapidly since the first successful application of SESAM in 1992 [6]. Up to now, besides the semiconductor saturable absorber mirrors (SESAMs) [6, 7], transition-metal-ion-doped SAs (mainly including Cr4+-doped host crystals like Cr:YAG [8, 9] and Cr:YSO [10], and also Cr2+-, V3+- and Co2+-doped crystals like Cr:ZnS [11], V:YAG [12] and Co:LMA [13]), carbon nanotubes [14, 15], and graphene [16–20], some graphene-like two-dimensional (2D) materials have been proven to exhibit an ultrafast saturable absorption property, such as topological isolators (TIs) (including Bi2Se3, Sb2Te3, and Bi2Te3) [21–23], transition metal dichalcogenides (TMDs) (like tungsten disulfide and molybdenum disulfide) [24–35], and black phosphorus [36–38]. These 2D materials not only possess broad operation wavelength band like graphene but also have unique thickness-dependent band-gap and more excellent nonlinear optical response than graphene [24, 25]. Therefore, the new SAs based on 2D nanomaterials have been a hot topic. In the last two years, in order to pursue outstanding performances in the modulation depth, transmittance, and low cost aspects of SAs, studies on emerging saturable absorption materials still go on.
As a kind of transition metal oxide with high conductivity and metamagnetism, ferroferric-oxide (Fe3O4) bulks previously just act as the raw materials of telecommunications equipment, welding materials etc [39]. In recent years, with the rapid development of nanotechnology, the remarkable physical properties of Fe3O4 nanoparticles capture researchers’ significant attention. Actually, the FONP possesses an inverse spinel struction which consists of alternating accumulation between octahedra and tetrahedra-octahedral layers [39,40]. Based on the cubic structure, FONPs have higher specific area and wider variety of surface functionalization than their bulks [41]. Also, the magnetic property of Fe3O4 will transform from metamagnetism to superparamagnetism as the bulks shrink down to nanoparticles, and its high conductivity will turn into semi-conductive property [39]. In addition, FONPs exhibit high field irreversibility and extra anisotropy [39–42]. Based on these characteristics, FONPs have some relatively mature electromagnetic application in high gradient magnetic separation, high density magnetic recording, magnetic resonance imaging, and drug delivery fields [43–45]. Very recently, FONPs are found to be promising for optical applications. As a class of semiconductor, FONPs own the size-dependent bandgap and a large intrinsic third-order optical nonlinear susceptibility of 4.0 × 10−10 esu [42, 46], which contribute to the broad wavelength coverage and large nonlinear optical response over the waveband. The corresponding response time ranges from 18 to 30 picoseconds [39, 47]. In 2016, X. Bai et al. proved the nonlinear optical response of FONPs and reported on the first passively Q-switched erbium-doped fiber laser with FONPs SA [49]. In the same year, D. Mao et al. fabricated two kinds of FONPs SAs by embedding the nanoparticles into the PVA and PI [50]. Later, Y. Chen et al. reported the 613 ns single-wavelength and multiwavelength Q-switched fiber lasers using Fe3O4 nanoparticles [51]. Three successful applications verify the FONPs SA’s effectiveness for 1.5 µm passively Q-switched fiber lasers. Meanwhile, it enlightens us to use it in 1 µm SSLs.
In this paper, we prepare a kind of FONPs solution SA with the modulation depth of 2.49% at 1 µm and apply it in a passively Q-switched Nd:YVO4 laser for the first time. The shortest obtained pulse duration is 53 ns corresponding to repetition rate of 576.4 kHz. The average output power is 104 mW. The pulse energy and peak power are calculated to be 180 nJ and 3.53 W, respectively. The results experimentally confirm the promising applications of FONPs SA in pulsed SSLs.
2. Preparation and characterization of FONPs SA
The raw material of FONPs used in our work are the water-based ferrofluids (MF: Ferrotec EMG 509, pH = 8-9), which are the stable black-brown colloidal suspensions of Fe3O4 magnetic particles. In the ferrofluids, the nanoparticles with average diameter of about 10 nm are coated with anionic surfactant to prevent aggregation from Van der Waals attraction and facilitate suspension in deionized water or water soluble dispersant. Taking into account the stringent requirement of solid-state lasers for insert loss of saturable devices, the concentration of FONPs solution is diluted to 0.05 mg/mL. The diluted FONPs solution presents rust brown color as shown in Fig. 1(a). Figure 1(b) displays the scanning electron microscope image of FONPs. The lateral sizes of nanoparticles are measured to be ~18 nm, which is slightly larger than the given average diameter of 10 nm. This is attributed to the anionic surfactant coating film and random agglomeration of Fe3O4 nanoparticles.
Fig. 1 (a) The suspension of the Fe3O4 nanoparticles. (b) Scanning electron microscope image of FONPs.
The FONPs suspension is injected in a 1 mm thickness quartz cell for use. The linear transmission spectra and nonlinear optical absorption of the sample are respectively measured by an optical spectrometer scanning and a home-made picosecond pulsed Nd:YVO4 laser at 1064 nm. The measured linear optical transparency is illustrated in Fig. 2(a). It can be seen that the transparency curve emerge a sharp rise when laser wavelength incessantly varies from 300 to 700 nm and then maintain a very flat profile at a level of ~85.3% ± 0.7% in the wavelength range of 700~1100 nm. Compared with other reported FONPs SAs, our solution SA has better optical transparency in the selected 1 µm spectral region which is particularly applicable to solid state laser. Ruled out the interference from absorptions of solvent solution and quartz cell, the linear optical absorption of pure FONPs at 1064 nm is 5.5%. Due to the intrinsic third-order optical nonlinear susceptibility, FONPs solution SA displays a nonlinear saturable absorption property. As shown in Fig. 2(c), we investigate the nonlinear optical absorption of the FONPs solution SA by using a twin-detector measurement technique [49]. The laser source is a home-made mode-locked Nd:YVO4 solid state laser with the central wavelength of 1064 nm, pulse duration of 25 ps, and repetition rate of 124 MHz. The laser beam is split into two equal powered arms by a beam splitter mirror. One arm is used for power-dependent transmission measurement of SA and the other one acts as a reference. The variable optical attenuator (VOA) is used to continuously alter the test power. With the increase of input power, a series of transmittance data are recorded and then fitted by the formula , as Fig. 2(b) shows, where T(I) is the transmission, ΔT is the modulation depth, I is the input intensity, Isat is the saturation intensity, and Tns is the non-saturable loss. Based on the experiment data fitting and the formula above, the modulation depth (ΔT), saturable intensity (Isat), and non-saturable loss (Tns) are determined to be 2.49%, 11.57 KW/cm2, and 11.73%, respectively.
Fig. 2 (a) Linear transmission spectra and (b) nonlinear optical absorption of FONPs solution SA. (c) Schematic diagram of experiment setup of nonlinear optical absorption measurement. (d) Damage threshold of FONPs SA.
Finally, we measure the damage threshold of FONPs solution SA by using the same twin-detector measurement technique in Fig. 2(c). A typical measurement trace is exhibited in Fig. 2(d). It is noted that when the input peak power intensity is higher than 83.2 KW/cm2, the transmission drop is clearly observed. Therefore, the damage threshold of FONPs solution SA is estimated to be 83.2 KW/cm2.
3. Experimental setup
Figure 3 shows the schematic of passively Q-switched Nd:YVO4 laser structure. We can see that an 808 nm fiber-coupled diode laser (LD) is used to pump an a-cut Nd:YVO4 gain crystal after shaping by an optical focusing system. The maximum output power of LD is 30 W and the beam waist of pump laser is 200 μm with a numerical aperture of 0.22. The optical focusing system consists of two focus lens with curvature ratio of 1:2. The laser beam size can be doubled through it. The gain media with a Nd3+ concentration of 0.7 at.% is cut into dimensions of 17 mm in length and 3 × 3 mm2 in cross-section, both of its light-passing faces are coated for 808 nm and 1064 nm anti-reflection (AR) films. To ensure appropriate spot size in the crystal and on the SA, a V-folded cavity with the length of 9.1 cm is designed. In the cavity, two dichroic mirrors M1 (R = ∞) and M2 (R = 50 mm) are coated with the films of 808 nm high transmission (HT) and 1064 nm high reflection (HR). The SA is a 1 mm thickness quartz cell filled with FONPs solution. One outer surface of quartz cell has the transmission of 5% at 1064 nm so that the cell can act as an output coupler (OC) as well as a SA. With the assistance of focusing effect from folding mirror M2, we can change the spot size on SA by altering the gap between SA and M2. Based on ABCD propagation matrix theory, simulation results of laser beam waist size in the gain medium and on the surface of SA are ~400 μm and ~81 μm, which match the pump laser mode and meet the absorption of FONPs SA well. In order to effectively alleviate the thermal lensing effect, the crystal is tightly mounted in a water-cooled copper block maintained at 16°C by a cool-water machine [4, 5].
4. Experimental results and discussions
In the experiments, the prepared FONPs solution SA is inserted in the designed V-folded cavity SSLs. By slightly tuning cavity mirrors, we obtain a stable passive Q-switching (QS) operation when the pump power exceeds 5 W. The corresponding average output power is 20 mW. The pulse duration and repetition rate are measured to be 130 ns and 204.1 kHz by a fast photodetector (DET10 A/M, Thorlabs, Inc., U.S.) and a digital oscilloscope (TDS5000B, Tektronix, Inc., U.S.), respectively. When pump power increases to 11 W, the Q-switched operation could be still maintained, while the pulses have slight jitter under high pump power of above 10 W. The jitter is mainly attributed to the degeneration of SA caused by laser-induced heat accumulation [48]. During the passively Q-switched experiments, the pulse duration and repetition rate are different under different pump power. Figure 4(a) shows three groups of pulse trains corresponding to the pump power of 5 W, 8 W, and 11 W. It is noted that the higher the pump power, the more the pulses generated and the narrower the pulse duration by comparing three groups of pulse trains. In addition, Fig. 4(b) indicates that the narrowest pulse of 53 ns is achieved under the pump power of 11 W. The precise evolutions of pulse width and repetition rate under per watt pump power are recorded, as displayed in Fig. 4(c). In general, the repetition rate of Q-switched pulses increases continuously from 204.1 to 576.4 kHz with the augment of pump power, while the single pulse duration decreases from 130 to 53 ns. The variation trends are in accord with the passive Q-switching principle [4].
Fig. 4 (a) The pulse trains of and (b) single pulse profiles of the Q-switched laser under the pump power of 5 W, 8 W and 11 W. (c) Evolutions of the repetition rate and of the pulse duration with the pump power. (d) The average output power of CW and of QS versus the pump power. (e) The single pulse energy and peak power curves. (f) The emission spectrums of CW and QS operations.
As a comparison, the average output power of continuous wave (CW) laser without SA and QS laser with SA are investigated in the same cavity structure. During the CW laser experiments, we have not observed any pulse generation. The QS pulses emerge only when we insert the FONPs SA in the laser cavity, confirming that it is the saturable absorption property of FONPs SA that operates for the generation of QS pulses. Figure 4(d) depicts the variations of CW laser output power and QS laser power versus pump power. On the whole, it is observed that the average output power of CW and QS increases approximately linearly versus pump power, respectively. However, the slope efficiency of QS laser is only 2%, and the gap to the 50% slope efficiency of CW laser is obvious. Such gap is mainly owing to the insertion loss of the FONPs solution SA. In addition, unlike the CW laser, the average output power of QS laser increases in the beginning and then abnormally falls when the pump power exceeds 10 W, which can be due to the optical limiting and the degeneration of SA caused by laser-induced heat accumulation [48, 50]. The maximum average output power approaches 121 mW under 10 W pump power. When the pump power exceeds 11 W, the FONPs SA is damaged by heat and the QS operation evolves into CW operation. The single pulse energy and peak power of QS pulses versus pump power are presented by Fig. 4(e). The largest single pulse energy of 253 nJ and highest peak power of 4.3 W are achieved when the pump power reaches 9 W. Because of SA’s degeneration under high power laser irradiation, the values of pulse energy and peak power decrease when the pump power is above 9 W. For the operation state of 53 ns shortest pulse, the single pulse energy and peak power are 180 nJ and 3.53 W, respectively. The CW and QS laser spectrums are measured by an optical spectrum analyzer (YOKOGAWA, AQ6370D) with the resolution of 0.02 nm. As shown in Fig. 4(f), the central wavelength of QS laser is at 1064.34 nm with a 3-dB bandwidth of 0.05 nm. Compared to the CW operation, the spectrum of QS is slightly broadened and the emission peak shifts to a longer wavelength. In addtion, the spectrum of the QS is a smooth Gassian-like curve without any other component. Moreover, during the experiments, we measure the spectrum of QS laser in a large scale of 580 nm (770~1350 nm) and no other emission peak is observed. So, we can confirm that there is no direct current power in our QS laser operation.
Based on the extraordinary semi-conductive property of FONPs materials, we experimentally confirm their nonlinear absorption property at 1 µm waveband for the first time. The modulation depth, saturable intensity, non-saturable loss, and damage threshold of FONPs solution SA are measured to be 2.49%, 11.57 KW/cm2, 11.73%, and 83.2 KW/cm2, respectively. Compared to the FONPs film SAs [49–51], our FONPs solution SA has lower saturable intensity, which can significantly reduce the threshold of passively Q-switched laser. Based on the low non-saturable loss and linear optical absorption, one can see that the solution SA possesses better optical transparency, which makes it be more applicable to the high power solid state laser. By the comparative test executed in the Nd:YVO4 laser cavity with FONPs and without FONPs, one can conclude that the FONPs induce a passive Q-switched operation described as follows: with the increasing of pump power, pulse duration of Q-switching drops from 130 to 53 ns, while the repetition rate raises from 204.1 to 576.4 kHz. The output power, single pulse energy, and peak power corresponding to 53 ns pulse are 104 mW, 0.18 µJ, and 3.53 W, respectively. It is the first time to successfully apply the FONPs SA in a Nd:YVO4 SSL. Besides, we obtain the super-100 mW average output power and super-4 W peak power. Additionally, due to the relatively high intracavity power and the short cavity, both 53 ns and 576.4 kHz are also the best results so far among pulsed lasers based on FONPs SA (shown in Table 1). Our results indicate the feasibility of FONPs SAs in Q-switching solid state lasers and their potential photonics applications in mode locking by enhancing the thermal damage resistance of FONP-based SA and optimizing cavity structure in future work.
Table 1. Development of pulsed laser based on FONPs SAs
5. Conclusion
In summary, we prepare a kind of FONPs solution SA with the modulation depth of 2.49%, the saturable intensity of 11.57 KW/cm2, and the non-saturable loss of 11.73%. By employing the SA to a diode end-pumped Nd:YVO4 laser, we achieve a 53 ns passively Q-switching with the high average power of 104 mW and the high peak power of 3.53 W. The corresponding repetition rate and the single pulse energy are measured to be 576.4 kHz and 180 nJ. The results experimentally indicate that FONPs can be a kind of suitable SA for generation of nanosecond pulse in solid state lasers.
Funding
Central University Special Fund Basic Research and Operating Expenses (GK201702005); National Natural Science Foundation of China (61378024).
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