All-solid-state passively mode-locked lasers have received wide attention due to their broad applications in industrial processing, spectroscopy, and medicine[
Chinese Optics Letters, Volume. 19, Issue 9, 091403(2021)
Theoretical analysis of periodically poled LiNbO3 nonlinear mirror and its application in a passively mode-locked Nd:YSAG laser
In this paper, we report a passively mode-locked
1. Introduction
All-solid-state passively mode-locked lasers have received wide attention due to their broad applications in industrial processing, spectroscopy, and medicine[
Recently, polycrystalline ceramics as laser gain media have several advantages over conventional single crystals, since they have a prolonged fluorescence lifetime and a broadened fluorescence spectrum[
Compared with the SA, the advantage of mode-locking based on intracavity frequency doubling is that it has higher damage threshold and output power. The periodically poled (PPLN) superlattice based on quasi-phase-matching is suitable for mode locking due to its relatively high nonlinear coefficient and no walk-off effect. Several research groups have demonstrated mode locking with periodically poled superlattices. In 2005, Holmgren et al. reported the mode-locked laser using periodically poled (PPKTP). The mode-locked pulse width is 2.8 ps, and the spectral bandwidth is 0.6 nm[
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2. Theoretical Analysis
The NLM contains a PPLN superlattice as a nonlinear crystal and an output coupler (OC), which has partial reflectivity for the fundamental wave (FW) and high reflectivity for the second harmonic (SH), as shown in Fig. 1(a). The extended nonlinear Schrodinger equation (NLSE) including the effects of dispersion and Kerr nonlinearity is used to analyze to performance of the NLM. In the slowly varying envelope approximation and in absence of diffraction, the NLSE of the FW and SH can be described as follows[
Figure 1.(a) Diagram of nonlinear mirror. (b) Nonlinear reflectivity dependence on intracavity peak intensity.
Here, is the nonlinear reflection, is the modulation depth, is the incident intensity, is the saturable intensity, and is the nonsaturable loss. The nonlinear coupled-wave item in Eq. (1) plays an important role when the NLM acts as an effective SA. As shown in Fig. 1(b), nonlinear reflectivity varies when is set to have different values. At higher , the effective SA is prone to be saturated at lower intracavity intensity. On the other hand, if the superlattice is of poor poling quality, it leads to lower , and NLM mode locking will have a relatively high self-starting threshold. After fitting the curves using Eq. (2), the modulation depth, saturable intensity, and nonsaturable loss were shown in Table 1. The modulation depth decreases, but saturable intensity and nonsaturable loss increase with decreasing . The increasing saturable intensity indicates that the MLN mode locking will have a high threshold. It is worth noting that the modulation depth, saturable intensity, and nonsaturable loss are 8.8%, , and 0.3%, respectively, when is set to be 14 pm/V, which corresponds to perfect poling of the superlattice with a duty cycle of 1:1.
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3. Material and Methods
The mode-locking performance of the Nd:YSAG laser was investigated by using a standard Z-type cavity in the experiment. Figure 2 shows the diagram of the experimental setup of the mode-locked laser. The length, width, and height of the Nd:YSAG crystal was . It was installed in a copper bracket cooled by water circulation, and the temperature was maintained at 14°C. A fiber-coupled diode laser delivered 30 W pump light, and the central emission wavelength was 808 nm. An optical coupling system was utilized to yield a beam the same as a pump beam onto the laser crystal, where the beam radius is 200 µm. M1 was a flat mirror with an antireflection (AR) coating () at 808 nm and a highly reflective coating () at 1.06 µm. M2 (radius of curvature: 500 mm) and M3 (radius of curvature: 200 mm) were plane-concave mirrors. Both of them were coated with high reflectivity () at 1030–1080 nm. The OC M4 was a dichroic mirror that had partial reflectivity () for FW and high reflectivity for the SH. The length of the PPLN superlattice was 5 mm. The period of the PPLN was 6.92 µm. In the experiment, the PPLN crystal was put in an oven, and the oven was located near the OC M4. In order to obtain a linearly polarized output beam, we inserted a Brewster polarization plate between the plane-concave mirrors M2 and M3. The total length of the resonant cavity was about 1.45 m. We calculated that the beam radii on the Nd:YSAG crystal and PPLN crystal were 190 µm and 80 µm, respectively.
Figure 2.Diagram of mode-locked Nd:YSAG laser (M1, flat mirror; M2, M3, plane-concave mirror; M4, output coupler; B, Brewster polarization plate; dotted rectangle is nonlinear mirror).
4. Experimental Results and Discussion
First, the PPLN was not inserted into the cavity, and the characteristics of the CW laser were studied. The pump power threshold was 1.5 W, and the output power increased linearly as the pump power increased. As shown in Fig. 3, the output average power reaches 930 mW when the pump power increases to 6.5 W. The corresponding slope efficiency is 18.3%.
Figure 3.Average output power dependence on pump power in the CW and CWML regime.
Then, the PPLN was inserted into the resonant cavity to study the performance of the CWML. The FW would be converted into SH by frequency doubling when it propagated through the PPLN. In order to improve the SH conversion efficiency, the temperature of the PPLN crystal was adjusted to achieve a phase-matching point. The optimized temperature for frequency doubling was controlled at about 20°C. As shown in Fig. 3, CW mode locking occurs when the pump power increased to 5 W. The cavity delivered 710 mW average output power with a slope efficiency of 14.6 % under the pump power of 6.5 W.
The pulse sequence was detected by a Si detector (Thorlabs, DET025AFC). The interval between adjacent pulses was 9.8 ns, which was in good agreement with the calculated roundtrip time by using a cavity length of 1.45 m, as shown in Fig. 4(a). Figure 4(b) shows the pulse sequence trace at 500 µs time scale. A digital oscilloscope (LeCroy, HDO4104A), which had a spectrum analyzing function, was used to measure radio frequency (RF) waveform, as shown in Fig. 5. The resolution bandwidth (RBW) was set to be 2 kHz within a span of 3.5 MHz. The fundamental central frequency was 101.7 MHz. The signal-to-noise ratio of the pulse was 45 dB. An optical spectrum analyzer (Avantes, AVASPEC-3648-USB2) was used to measure the optical spectrum. As shown in Fig. 6, there are two peaks oscillating in the cavity, 1061 nm and 1063.5 nm, respectively. To determine which wavelength participates in the mode-locking process, we made the analysis as follows:
Figure 4.(a) Mode-locked pulse sequence at 100 ns time scale. (b) Mode-locked pulse sequence at 500 µs time scale.
Figure 5.Radio frequency waveform of CWML laser.
Figure 6.Optical spectrum of mode-locked Nd:YSAG laser.
Figure 7.Second harmonic efficiency at different wavelengths.
The mode-locked pulse width can be estimated according to the following method. The full width at half-maximum (FWHM) of the spectrum is 1.6 nm at the central wavelength of 1061 nm. According to the time bandwidth product of a pulse with a Gaussian curve (), we calculated the pulse duration to be 1 ps. However, the pulse will be broadened due to the GVM in the PPLN. The GVM of the FW and SH after passing the PPLN twice is 1.6 ps/mm. In the experiment, the length of PPLN is 5 mm, so the SH is relatively delayed to the FW by 8 ps. Thus, the final minimum pulse width is estimated to be 9 ps.
It is worth noting that the minimum pulse intensity of the intracavity pulse is , assuming that the conversion of SH generation is a small signal approximation. It is higher than the theoretical saturation intensity of , which corresponds to a maximum of 14 pm/V. Therefore, it indicates that the PPLN superlattice is of high poling quality, and the NLM could effectively act as an SA in our experiment.
5. Conclusions
In conclusion, we report a passively mode-locked Nd:YSAG laser based on a PPLN superlattice NLM. NLM mode locking was theoretically analyzed. The modulation depth of nonlinear reflectivity of the NLM was approximately 8.8%. Optical performances of the mode-locked laser including output power, RF spectrum, and optical spectrum were experimentally investigated. An average output power of 710 mW with a slope efficiency of 14.6% was obtained at the pump power of 6.5 W. The repetition rate was 101.7 MHz, and the signal-to-noise ratio of the mode-locked pulse was 45 dB. The mode-locked pulse width was approximately 9 ps.
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Lina Zhao, Fangxin Cai, Luyang Tong, Ye Yuan, Wenyu Zhang, Yangjian Cai, "Theoretical analysis of periodically poled LiNbO3 nonlinear mirror and its application in a passively mode-locked Nd:YSAG laser," Chin. Opt. Lett. 19, 091403 (2021)
Category: Lasers, Optical Amplifiers, and Laser Optics
Received: Nov. 23, 2020
Accepted: Mar. 4, 2021
Published Online: Jun. 15, 2021
The Author Email: Lina Zhao (lnzhao@sdnu.edu.cn), Yangjian Cai (yangjiancai@suda.edu.cn)