1. Introduction

High power 1.3 µm pulse laser has wide applications in optical fiber communication, laser medicine, optical sensing, and pump source of mid-infrared optical parametric oscillation, because this wavelength was located at the low intrinsic loss windows of the silica fiber and has a weak dispersion effect [1]. Q-switched and mode-locked laser technologies are the main paths to produce high-power pulse laser [2]. Compared with actively modulated lasers, passively modulated lasers with saturable absorbers such as passively Q-switched (PQS) laser and passively mode-locked (PML) laser, have the merits of low cost and simple structure without the need for high-voltage and RF drivers [3,4]. In addition, neodymium (Nd) doped vanadate crystals have been proved to be excellent 1.3 µm laser mediums, based on the possible ⁴F_3/2→ ⁴I_13/2 transitions of Nd³⁺ ions [5,6].

In recent years, ranging from graphene, transition metal dichalcogenides (TMDs), topological insulators (TIs) to layered double hydroxides (LDHs), nanomaterials have been developed and successfully employed as SAs in PQS and PML lasers, respectively [7–14]. As a typical member of LDHs, Ni-based LDHs have been demonstrated to be promising candidates for PQS 2 µm laser by our group. Compared to graphene, TMDs, TIs etc., NiCo-LDH exhibit more diverse electronic–photon interaction due to the natural interaction between anions and photons and the interactions between functionalized NiCo-LDH nanolayers and photons. Using NiCo-LDH as SA, a stable PQS laser operation with a pulse width of 322 ns and a repetition frequency of 119 kHz was realized, which was the shortest PQS pulse width of the Tm:YAG crystal and ceramic lasers with two-dimensional material SAs [15]. Unfortunately, the PML laser modulation characteristics of Ni-based LDHs have not been investigated yet and their PQS laser performance at 1.3 µm also needs to be further explored.

Owing to smaller stimulated emission cross section and larger fluorescence bandwidth, Nd:Lu_xY_1-xVO₄ mixed crystal was identified as an outstanding laser medium to generate efficient PQS and PML laser [4,16]. T. Li et al. demonstrated a 5.1 ps PML Nd:Lu_0.15Y_0.85VO₄ laser operation at 1065 nm with SESAM [17]. As for 1.3 µm, a PQS Nd:Lu_0.5Y_0.5VO₄ laser with Co²⁺:LMA SA was achieved with a minimum pulse width of 154.8 ns and a pulse repetition frequency of 163.7 kHz [18]. In 2015, W. Qiao et al. realized an efficient 1.34 µm PML laser performance of Nd:Lu_0.15Y_0.85VO₄ mixed crystal with SESAM, resulting in a shortest pulse width of 15.2 ps and a repetition rate of 32.8 MHz [19].

In this paper, NiCo-LDH SA was successfully fabricated and applied in the 1.3 µm laser modulation experiments of Nd:Lu_0.15Y_0.85VO₄ mixed crystal. In the PQS experiment, a narrowest pulse width of 52 ns was obtained, with a highest repetition frequency of 1.18 MHz. The corresponding single pulse energy was 0.72 µJ, and the peak power was 13.89 W. Meanwhile, a passively Q-switched mode-locked (QML) pulse with a pulse width of 25 ps and a repetition frequency of 134 MHz was also obtained at 1.34 µm. As far as we know, this is the first time for the Ni-based LDHs to be adopted as a PML laser modulation and also firstly be investigated in the 1.3 µm wavelength laser.

2. Fabrication and characterization of the NiCo-LDH SA

The NiCo-LDH powder was prepared by the one-step hydrothermal method, which is similar to the Ref. [15]. In order to fabricate a NiCo-LDH SA, we added 50 mg of NiCo-LDH powder into a 10 mL centrifuge tube and filled it with alcohol. Then, the centrifuge tube was put into an ultrasonic cleaner for ultrasonic treatment for 12 h, and centrifuged at 2000 rpm for 15 min. Finally, we collected 200 µL of the supernatant, and dropped it over the quartz substrate. After being dried at room temperature, the NiCo-LDH SA was prepared successfully.

The Raman spectrum of NiCo-LDH was exhibited in Fig. 1 (a), which was excited by a 633 nm laser source. The sharp peak located at 526 cm^-1 was due to the vibration of Ni-Co and the broadband near 3611 cm^-1 proved the O-H stretching mode. The peak at 1068 cm^-1 originated from the ${\mathrm{\nu }_{\textrm{C} - \textrm{O}}}$ vibration mode of methanol molecules [20]. The element composition was measured by Energy Dispersive Spectrometer (EDS) (SU8010, HITACHI). In Fig. 1 (b), the proportion of the composed four elements of C, O, Ni, and Co was illustrated. As respective images of NiCo-LDH, Fig. 1 (c) and (d) was the scanning electron microscope (SEM) and transmission electron microscope (TEM) images of NiCo-LDH, where the ribbon-like nanosheet structure was depicted. The measurement result of the atomic force microscope (AFM) image was shown in Fig. 1 (e), and the corresponding height was given in Fig. 1 (f). The results revealed that the thickness of NiCo-LDH nanosheet was about 12 nm.

 

Fig. 1. (a) Raman spectra of NiCo-LDH powders; (b) EDS image of NiCo-LDH; inset: Percentage of elements in NiCo-LDH; (c) SEM image of NiCo-LDH; (d) TEM image of NiCo-LDH; (e) AFM image, and (f) height variations in the region marked in AFM image of NiCo-LDH.

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Figure 2 (a) displayed the light absorption characteristics of NiCo-LDH, tested by UV-Vis-NIR spectrophotometer. The light absorption ratio of NiCo-LDH at 1343 nm is 20.1%. In order to investigate the nonlinear effect of NiCo-LDH SA, a self-made pulse laser with a wavelength of 1343 nm was used to measure the light transmittance of NiCo-LDH SA. The pulse width of the pulse laser was 500 µs and the repetition frequency was 1 kHz. The result was exhibited in Fig. 2 (b), and the experimental data was fitted by the following formula [21]:

In the Eq. (1), A is the absorption of the SA, $\Delta R$ is the modulation depth, I is the energy intensity, ${I_s}$ is the saturable fluence, and ${T_{ns}}$ is the unsaturated loss. According to the fitting results, the modulation depth, saturable fluence and unsaturated loss of NiCo-LDH SA are 5.9%, 0.53 mJ/cm², and 13.6%, respectively.

 

Fig. 2. (a) The linear absorption of NiCo-LDH; (b) The nonlinear absorption of NiCo-LDH SA.

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3. Passively Q-switched Nd:Lu_0.15Y_0.85VO₄ laser at 1.34 µm

The PQS experimental schematic diagram was presented in Fig. 3. The pump source was a fiber-coupled semiconductor laser with a central wavelength of 808 nm, and the fiber core diameter was 200 µm. Through a 1:1 coupling system, the pump laser was focused into the laser medium. M1 was the input flat mirror, with high transmittance (HT) at 808 nm and high reflectivity (HR) at 1343 nm. M2 was a concave output mirror with a 100 mm curvature radius, with two transmittances of 10% and 5% at 1343 nm. The size of the gain medium Nd:Lu_0.15Y_0.85VO₄ was 3×3×10 mm with a Nd³⁺ doping concentration of 0.38 at.%, which was grown by the Czochralski method and cut along the a axis. In the experiment, the crystal was wrapped in indium foil and placed in a copper heat sink with a water-cooled temperature of 12 °C. The average output power was measured by the power meter (THORLABS, s302c). The output PQS pulse was recorded by a photoelectric probe (EOT, ET-3000) and an oscilloscope (Agilent Technologies, DSO-X3104A). And the laser spectrum was detected by a spectrometer (A.P.E, Germany).

 

Fig. 3. Passively Q-switched 1.34 µm laser experimental device with NiCo-LDH SA.

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The relationship between CW laser power and PQS laser power versus the absorption pump power was described in Fig. 4(a) and (b), respectively. When the absorbed pump power was 6.96 W and the transmittance of the M1 was T = 10%, the CW output power reached the maximum value of 1.38 W, corresponding to a light-to-light conversion efficiency of 19.9% and a slope efficiency of 20.3%. The central spectrum was 1343.60 nm with a full width at half-maximum (FWHM) of 0.62 nm. When the transmittance T = 5%, the maximum output power was 1.23W, with a light-to-light conversion efficiency and a slope efficiency were17.7% and 18.7%, respectively. By inserting the SA into cavity, the PQS laser operation was obtained. When the transmittance was T = 10% and T = 5%, the maximum PQS output power obtained was 0.85W and 0.75W, respectively. The corresponding optical efficiency was 12.2% and 10.8%, and the slope efficiency was 13.1% and 12.3%. Due to the inserting loss of the SA, the central spectrum wavelength of 1342.9 nm had a blue-shift phenomenon with a narrower FWHM of 0.4 nm.

 

Fig. 4. Output power versus absorbed pump power of (a) the CW laser; inset: CW lasing spectrum; (b) the PQS laser; inset: PQS lasing spectrum.

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The PQS Nd:Lu_0.15Y_0.85VO₄ laser characteristics at 1.34 µm was given in Fig. 5. As can be seen from Fig. 5(a) and (b), as the rising of absorbed pump power, the repetition frequency gradually increased, while the pulse width gradually became narrower. When the transmittance was T = 10%, the maximum repetition frequency was 1.18 MHz, and the narrowest pulse width was 52 ns. Replaced by T = 5%, a narrowest pulse of 54 ns was gained with a maximum repetition frequency of 1.06 MHz. Figure 5(c) and (d) was the calculated single pulse energy and peak power severally. When the transmittance T = 10%, the maximum single pulse energy was 0.72 µJ, and the corresponding peak power was 13.89 W.

 

Fig. 5. (a) Repetition rate; (b) Pulse width; (c) Pulse energy and (d) Peak power versus absorbed pump power for PQS laser.

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Figure 6 depicted the pulse sequence and the profile of a single pulse under the output mirror with different transmittances. The pulse sequence was obtained under the maximum absorption pump power of 6.69 W. The time grid of the multi-pulse sequence was set to be 10 µs per division, and the time grid of the single pulse profile was 100 ns per division.

 

Fig. 6. Typical pulse trains and temporal pulse profiles with different output mirrors at maximum absorbed pump power.

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4. Passively Q-switched mode-locked Nd:Lu_0.15Y_0.85VO₄ laser at 1.34 µm

The PML Nd:Lu_0.15Y_0.85VO₄ laser schematic diagram was illustrated with a 2.24 m long Z-type optimum resonator in Fig. 7. The pump source, coupling system and laser crystal used in the experiment were the same as the above PQS experiment. M1, M3, and M4 was coated with HT at 808 nm and HR at 1343 nm. M1 and M4 were flat mirrors, and M3 was a concave mirror with a 500 mm radius of curvature. The output mirror M2 was a concave mirror with a 100 mm radius of curvature and a transmittance of 10% at 1343 nm. According to the ABCD matrix method, the fundamental mode radius inside the mixed crystal was calculated to be about 100 µm, while the fundamental mode radius at M4 was about 180 µm. To reduce inserting loss, the NiCo-LDH supernatant was directly dropped onto M4 to produce PML laser. The QML pulse was recorded by a photoelectric probe (1611, New Focus). The other detectors were the same as the PQS experiment.

 

Fig. 7. Passively Q-switched mode-locked 1.34 µm laser experimental device.

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The CW and QML output power of Nd:Lu_0.15Y_0.85VO₄ laser versus absorbed pump power was displayed in Fig. 8. When the absorbed pump power was 6.96 W, the CW output power was 1.56 W, leading to an optical efficiency of 22.4% and a slope efficiency of 25.4%. Meanwhile, for the QML laser, the maximum average output power was 1.10 W, and the optical efficiency and the slope efficiency were 15.8% and 20.3% severally. The inset of Fig. 8 was the output spectrum of QML laser. The central wavelength of the spectrum was 1343.53 nm and the FWHM was 0.63 nm. Regretfully, there was no CW PML laser occurrence throughout the whole experiment process. It is probably because of low energy density inside the cavity and thermal-lens effect of the mixed crystal. On the one hand, theoretical and experimental results reveal that CW PML can be triggered only when the power density at SA reaches a certain threshold [22]. Obviously, the power density at M4 in our experiment was less than density threshold required for CW PML. On the other hand, as depicted in Fig. 8, there was power saturation phenomenon with the increase of pump power in the experiment, which indicates severe thermal lens effect of mixed crystal. And it also leads to unstable laser modulation and the inability to establish a CW PML [19]. Next, we will continue to explore the CW PML characteristics of NiCo-LDH SA.

 

Fig. 8. The output power versus absorbed pump power of CW and QML; inset: QML lasing spectrum.

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As can be seen from Fig. 9, the 1.34 µm QML pulse displayed a high modulation depth of 80%, and the QML pulse repetition frequency was 134 kHz, which matches with the equivalent cavity length of 2.24 m. The single pulse width can be estimated by the following formula [23,24]:

In the Eq. (2), t_r is the rise time of the real ML pulse; t_m is the measured rise time; t_o and t_p are the rise time of the oscilloscope and photoelectric probe, respectively. The pulse width is approximately 1.25 times of the rise time [23,24], which can be calculated to be 25 ps.

 

Fig. 9. QML pulse trains recorded in (a) 200 ns and (b) 10 ns per division time scales.

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5. Conclusion

In conclusion, the NiCo-LDH SA was successfully applied as 1.3 µm PQS and PML laser modulator for the first time, and the Nd:Lu_0.15Y_0.85VO₄ mixed crystal was also employed. A shortest PQS pulse width of 52 ns at 1342.9 nm was obtained with a repetition frequency of 1.18 MHz. For the PML laser, a QML pulse width of 25 ps with an average output power of 1.10 W was achieved, and the corresponding repetition frequency was 134 MHz. The results reveal that NiCo-LDH SA has a promising application prospect as 1.3 µm pulse laser modulators.