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Abstract
A promising Nd:YAG single-crystal fiber (SCF) was successfully fabricated by the laser-heated pedestal growth method. The compact passively Q-switched laser has been realized for the first time with a 794 nm laser diode pumping. Pulsed laser performances of this SCF were investigated in detail with a flat-flat resonator. Under the absorbed pump power of 2.50 W, the peak power of 274.24 W was obtained with 19.41 ns pulse duration width and 65.94 kHz repetition rate.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Compact nanosecond-magnitude pulsed laser has gradually become a desired product due to its small size, low cost and portability. Passive Q-switching is an important way to realize this kind of laser, which has been used in a wide range such as welding, medicine, material processing and laser storage [1–3]. As the foundation and core of laser technology development, gain medium has always been the focus of research. The development and improvement of gain medium also promotes the progress of solid-state laser [4–8]. Meanwhile, single-crystal fibers (SCFs) are the fiber-shaped monocrystalline materials. It not only has the excellent physical and chemical properties of bulk crystals, but also displays the excellent thermal dissipation efficiency of optical fibers [9,10]. Different from traditional glass fibers, SCFs have the advantages of low nonlinear effect and higher thermal conductivity. Among them, the low Brillouin gain coefficient makes the SCFs less affected by nonlinear effects such as Brillouin scattering. At the same time, the high thermal conductivity makes internal temperature gradient small. Thus, it is more conducive to the generation of superior performances laser [11–13]. In relation to the bulk single crystals, SCFs maintain the high aspect ratio of traditional fiber design, which is helpful for the thermal management of laser system. As gain medium, SCFs can also obtain a large gain bandwidth of mode spacing, that provide a new occasion for the implementation of micro-compact pulsed laser [14–17]. Especially, Nd:YAG SCF has admirable mechanical, physical and optical characteristics while it can produce laser in the band of 0.9 µm, 1.06 µm, 1.1 µm, 1.3 µm, 1.4 µm [18]. Therefore, it has a potential application prospect.
In recent years, the research of laser characteristics of Nd:YAG SCF laser has become a hot topic [19–22]. At present, Nd:YAG SCF has achieved high efficiency continuous-wave (CW) laser [23]. For all we know, acousto-optically Q-switched laser have been reported in the band of 0.9 µm and 1.4 µm [24–26]. However, acousto-optical Q-switching is not easy to form a pulsed laser with high repetition rate. At the same time, the acousto-optically Q-switching also need the radiofrequency driver and the structure is more complex that increase the costs of laser sources. It just so happens that passively Q-switching technique has the preponderance of compact structure, which is because that the cavity length of resonator is not restricted by the size of other external devices. Through this technique, short cavity length with low cost can be achieved. Therefore, it is very suitable for making small and medium power laser, and is convenient for practical application to satisfy the pursuit of small instrument [27–29]. Whereas, there is no related research on passively Q-switched laser at any wavelength band for this SCF.
In this paper, Nd:YAG SCF was grown by the method of laser-heated pedestal growth. Passively Q-switched laser was first demonstrated with Nd:YAG SCF. Pumped by a 794 nm laser diode, a stable pulsed laser with narrow pulse width and high repetition rate was realized ground on the mature Cr4+:YAG as saturable absorber (SA). The performance of passively Q-switched laser was systematically investigated. The measured pulse width was 19.41 ns, with the repetition rate of 65.94 kHz. The single pulse energy and peak power were calculated to be 5.32 µJ and 274.24 W. These results show the potential of Nd:YAG SCF in pulsed laser applications.
2. Spectra properties of Nd:YAG SCF
Nd:YAG SCF was successfully prepared by the laser-heated pedestal growth method which is a kind of miniature light float zone method. The advantage is that the heating zone is fixed by surface tension, as crucible is not used in the growth process which may reduce the contamination [30]. The Nd:YAG SCF sample was observed under a microscope. There was no obvious scattering inside the sample for the formation of microbubbles which proved that the crystallization quality of our material is relatively good. Absorption spectrum and emission spectrum are shown in Fig. 1(a) and Fig. 1(b), respectively. Figure 1(a) display that the SCF has a strong absorption in the range of visible light, so a laser output can be obtained by using a flashlamp pumping. However, because of the wide emission spectrum of the flash lamp, the laser crystal can’t completely absorb the pump light, resulting in low utilization rate. Compared with the flashlamp pumping, laser diode pumping can effectively improve the conversion efficiency and reduce the thermal effect of the laser, which is conducive to obtaining high-quality laser. Therefore, this SCF is particularly suitable for laser diode pumping. In addition, it can be seen from the absorption spectrum, the absorption peak at 794 nm is significantly higher than that at 808 nm. Thus, a 794 nm laser diode was selected as the pump source with a 200 µm core diameter and 0.22 numerical aperture. Additionally, it can be known from the Fig. 1(b) that Nd: YAG SCF has great advantages in realizing the pulsed laser at 1064 nm.
3. Experimental setup
Doping concentration of this SCF was 0.6 at.% with a diameter of 1 mm. To use the SCF as gain medium, an appropriate cutting length was needed to obtain more efficient laser output. The principle of cutting is to estimate the length of the sample based on its absorption spectrum. According to this formula:$l = \frac{{ln\frac{{{I_0}}}{I}}}{\alpha }$, here $\frac{{{I_0}}}{I}$ represents the absorption of pump light by the sample, α is the absorption coefficient, l is the length of the sample that we selected in our experiment. We can make a preliminary estimate that the length of our sample is suitable for 8 mm length. Therefore, we cut the original length to 8 mm. In order to characterize the laser transmission capability of SCF, the loss measurement is carried out by using the following formula:$\alpha \textrm{ = }\frac{1}{l}10lg\left( {\frac{{{P_{in}}}}{{{P_{out}}}}} \right)$, where Pin and Pout are the incident power and output power of the SCF, respectively. The seed source was a laser diode with central wavelength of 980 nm. As both ends of the SCF are polished and uncoated, the Fresnel reflection coefficient needs to be considered. Through calculation, the transmission loss coefficient is about 0.043 dB/cm. The main reasons for the relatively high transmission loss are as follows: on the one hand, because of the doping of Nd3+, SCF still has less absorption at 980 nm. On the other hand, the quality of SCF is still to be further improved, and the fiber cladding is not realized, resulting in more scattering loss.
The laser schematic is shown in Fig. 2. In order to effectively reduce the heat dissipation, the SCF was placed into the water-cooled copper block with 12 °C temperature during this experiment. Then the pump light was coupled into the SCF by a 1:1 focusing system. In order to make the laser structure more compact, a flat-flat resonant cavity was specially designed. Among the cavity mirrors, M1 was an input mirror with anti-reflection (AR) coated for 808 nm and high-reflection coated for the output laser wavelength. Output couplers (OCs) with transmissions of 5% and 10% were selected. The Cr4+:YAG-SA has a transmission of T = 95% at 1064 nm. By inserting it into the cavity, Q-switching operation was accomplished. The cavity length was almost 17.8 mm. Under the premise of considering the thermal lens effect, the beam radius of oscillating spot on Nd:YAG SCF and Cr4+:YAG-SA were 105.50 µm and 99 µm when the absorbed pump power was 2.50 W. Through the simulation calculation by ABCD propagation matrix, the oscillation spot radius at the SCF and SA match with the pump spot radius.
4. Results and discussion
At first, the characteristics of CW laser operation were analysed. By using a laser power meter (30 A-SH-V1, Israel, Ophir), the average output power was measured. In order to obtain better experimental results, the OCs with different transmissions (T = 5% and T = 10%) were used for comparison. The absorption efficiency of 0.6 at.% Nd:YAG SCF is 71.4% which was shown in the inset of Fig. 3. It can be seen that the absorbed pump power of SCF increases linearly with the change of incident pump power, which indicate that our material has a good absorption of the pump light. Respective at the absorbed pump power of 6.50 W and 6.80 W, the output power of 1.80 W and 2.17 W were obtained. Meanwhile the corresponding slope efficiency were 29.8% and 35.9% respectively. Laser performances of CW laser operation was shown in Fig. 3. Because the thermal effect in the SCF is too significant at higher power, we did not continue to increase the incident pump power which aims to protect our material. The SCF which we used has not been coated. The following, we will consider coating the Nd:YAG SCF with AR film at 1064 nm and high-transmission film at 794 nm. In addition, our material has defects in the growth process, so the optical properties are affected. Therefore, the research on the properties of pulsed laser can provide a reference for the further optimization of material. In this way, the loss caused by defects in material growth can be reduced, so that the average output power is expected to increase further.
Subsequently, the Cr4+:YAG-SA was inserted into the cavity after aligning CW laser to optimum state. Then we carefully adjust our resonator to obtained stable passively Q-switched laser operation. At the same time, the oscillation threshold of the laser was also increased with the SA insert, which also caused extra optical loss to the laser cavity. While the absorbed pump power was adjusted from 0.60 W to 2.50 W, laser operated in stable passively Q-switched status. Further increase the incident pump power, the pulse trains started to get a little unstable. Meanwhile, when the intracavity energy density was far larger than the saturation effect of Cr4+:YAG crystal, the SA was always in the saturation state, and eventually the laser turned out to be unmodulated. It can be observed that the output power was relatively stable under different incident pump power, which indicates that our pulsed laser always works in a stable state. Under the absorbed pump power of 2.50 W, the output power of T = 5% and T = 10% OCs were 306 mW and 351 mW, corresponding to slope efficiencies of 14.9% and 17.6%, respectively. Figure 4 depicts the output performances in the state of pulsed operation and both of them are linear relationship. The relatively low average output power mainly caused by thermal effect on Cr4+:YAG-SA. Moreover, due to the limitation of experimental conditions, no corresponding refrigeration device was installed on the SA. Therefore, when continued to increase the pump power thereafter, although the output power can still be increased, the pulse trains became no longer stable. To prevent our SA from being damaged, the incident pump power was no longer increased. The inset of Fig. 4 shows the corresponding laser spectrum of T = 10% which was measured by a spectrometer (Ava-spec 3648-USB2, Avantes, Netherlands) with measurement accuracy was 0.5 nm. The central wavelength was 1063.9 nm with full width at half maximum of 0.8 nm.
Fig. 4. Average output power of pulsed laser based on Cr4+:YAG-SA. Inset: T = 10% optical spectrum of pulsed laser.
In this experiment, with the absorbed pump power increases, the pulse width decreases, whereas the repetition rate increases, as shown in Fig. 5(a). With the absorbed pump power increased from 0.60 W to 2.50 W, the pulse width of the Nd:YAG SCF laser decreases drastically. This rapid decreasing trend also consistent with the typical phenomena that pulse width varies with the incident pump power [31,32]. The gradual increase of the pump power makes the inversion particles in the low Q-value state accumulate in the upper energy level of the gain medium. When the number of inverted particles accumulates to the maximum, the absorption of Cr4+:YAG-SA tends to be saturated. Drastic laser oscillations are set up inside the resonant cavity to converts particle energy into laser energy. A shorter pulse can be emitted in a much shorter time. While continuous increase the pump power, the density of inverted particles in upper energy level of Nd:YAG SCF increases gradually. The bleaching time of SA is shortened rapidly, which leads to the increase of the net gain coefficient of the optical cavity and resulting in a continuous increase in the pulse repetition rate. The single pulse energy and peak power with absorbed pump power were presented in Fig. 5(b), which were calculated by the formulas: $E = \frac{{{P_{ave}}}}{f}$ and ${P_{peak}} = \frac{E}{\tau }$, where E is the single pulse energy, Pave is the average output power, f is the repetition frequency, Ppeak is the peak power and τ is the pulse width. At the absorbed pump power of 2.50 W, the pulse width and the repetition rate were 19.41 ns and 65.94 kHz. The single pulse energy was 5.32 µJ, corresponding peak power was 274.24 W under the condition of T = 10% OC. Table 1 summarizes the detailed results with different transmissions of OCs.
Fig. 5. Variation of (a) pulse widths, repetition rates (b) single pulse energy, peak power with absorbed pump power.
Table 1. Performances of Nd:YAG SCF pulsed laser under different OCs.
Pulse trains were recorded with a 1-GHz digital oscilloscope and a fast InGaAs photodetector. From Fig. 6, the characteristic of neat pulse trains at a time scale of 400 µs/div indicates that our pulsed laser can still maintain stable operation at higher incident pump power. Meanwhile, this state can last for a long time, which indicates that it has more prospects in practical application. The results also show that when T = 10%, our experimental results are even better. Therefore, with this transmission, a laser beam quality analyzer (Spiricon-M2-200S-USB) was used to measure the laser beam quality factor M2 and spatial beam profile at an output power of 351 mW under the state of passively Q-switched, which were shown in Fig. 7. Since no refrigeration device was added to the SA, the thermal effect was relatively significant, which may affect the beam quality. Following the experimental conditions will be improved to enhance the beam quality.
5. Conclusion
The Nd3+-doped YAG SCF was successfully prepared and its pulsed laser characteristics were researched in this paper. Pumped by the 794 nm laser diode, a compact Q-switched 1064 nm Nd:YAG SCF laser was demonstrated firstly. Physical length of the laser cavity was only 17.8 mm, which is extremely beneficial for practical applications. Detailed pulsed laser performances were investigated experimentally. With 2.50 W absorbed pump power, a pulse width was 19.41 ns, whereas the repetition rate and highest peak power were 65.94 kHz and 274.24 W. These results indicate Nd:YAG SCF would be a promising gain medium. Higher power pulsed laser is expected to be realized by improving the growth quality of SCF and optimizing the cavity design.
Funding
National Natural Science Foundation of China (11974220, 61925508, 62005302); International Partnership Program of Chinese Academy of Sciences (121631KYSB20180045).
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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