1. Introduction

With ultra-short pulse duration and high peak power, diode-pumped solid-state intra-cavity green lasers have been widely employed in the fields of ophthalmology, information storage, and holography. Specially, green laser also plays a significant role in the fields of underwater communication and ocean exploration for the minimal water absorption in the blue-green wavelength range. This kind of 532 nm green laser can be simply obtained by inserting a nonlinear crystal into a simultaneous passive Q-switching and mode-locking (QML) solid-state laser which is operated at the fundamental wavelength of 1064 nm [1–6 ].

To obtain shorter pulse duration and higher peak power, many efforts have been made, such as Q-switching and continuous-wave mode-locking (CWML). The traditional Q-switching laser could produce nanosecond pulse duration with several or dozens of kilohertz repetition rate. Meanwhile the passive mode-locking laser can generate even shorter laser pulses from several picoseconds to hundreds of femtoseconds with the repetition rate from MHz to GHz, which correspond to nanojoule pulse energy [7–10 ]. The Q-switching or CWML could not simultaneously obtain shorter pulse duration and higher peak power, hence to obtain sub-nanosecond pulse duration with low repetition rate and high pulse energy, much attention should be paid.

Simultaneously Q-switching and mode-locking technology can generate pulse duration of sub-nanosecond. To obtain stable QML laser, dual-loss-modulated technology, simultaneously employing an active modulator [acoustic-optic (AO) modulator or electro-optic (EO) modulator] and a passive saturable absorber in the cavity, has been investigated by several groups [11–15 ]. Though the pulse durations rest with the active modulator and the absorption of saturable absorber, the repetition rate is under the control of active modulator, thus several kHz repetition rate could be obtained. In 2013, Zhang et al. reported single mode-locking pulse generation underneath the Q-switched envelope of the doubly QML green laser with EO and GaAs [16], and solid-state YVO₄/Nd:YVO₄/KTP sub-nanosecond green laser simultaneously employing EO and C-SESAM [17]. Low repetition rate sub-nanosecond pulse characteristics of Nd:Lu_0.5Y_0.5VO₄/KTP green laser with EO and MWCNT was investigated by Zhang et al. in 2015 [18], and the highest peak power of 1.022 MW was obtained by that time. In passively Q-switched and mode-locked Nd:GGG laser [19], Bi-doped GaAs generated QML pulses with larger average output power, shorter pulse width, and deeper modulation depth than GaAs. Our previous work showed that the multi-segment composite Nd:YVO₄/Nd:YVO₄/Nd:YVO₄ could effectively mitigate the thermal lens effect and obtained better laser performance when compared with the single crystal [20]. Thus to pursue higher peak power sub-nanosecond pulse, the combination of the composite crystal and Bi-GaAs is employed in our experiment. To the best of our knowledge, there is no report on the dual-loss-modulated QML multi-segment composite Nd:YVO₄/Nd:YVO₄/Nd:YVO₄/KTP green laser with EO and Bi-doped GaAs.

In this paper, by simultaneously employing EOM and Bi-doped GaAs or GaAs saturable absorbers, diode-pumped dual-loss-modulated QML multi-segment composite Nd:YVO₄/Nd:YVO₄/Nd:YVO₄/KTP green lasers are investigated. Only one mode-locking sub-nanosecond pulse underneath a Q-switching envelope is generated when the incident pump power is beyond a certain value. At 1 kHz repetition rate and the incident pump power of 11.13 W, the largest average output power of 0.445 W, the shortest pulse duration of 399 ps, and the highest peak power of 1.115 MW are obtained with EO and Bi-GaAs.

2. Experimental setup

The schematic setup is depicted in Fig. 1 . The pump source employed in this experiment is a commercial fiber-coupled laser diode, working at the maximum absorption wavelength (808 nm) of the Nd³⁺ ions (FAP system, COHERENT Inc., USA). The core size of the fiber is 400 μm in diameter, with a numerical aperture of 0.22. The pump light is launched into the crystal with 1:1 focal system. The laser host is an a-cut multi-segment composite Nd:YVO₄/Nd:YVO₄/Nd:YVO₄ crystal with increasingly Nd³⁺-doping concentrations (0.1 at.%, 0.5 at.%, and 1.0 at.%) which is fabricated by the thermal diffusion bonding technique with a dimensions of 3 × 3 × (4 + 3 + 3) mm³. The pump facet of laser host is antireflection (AR)-coated at 808 and 1064 nm, while the other facet is AR-coated at 1064 nm. The frequency-doubling crystal KTiOPO₄ (KTP), with the dimensions of 3 × 3 × 8 mm³, is cut for type-II phase matching at 1064 nm (θ = 90°, φ = 23.2°) and AR coated at 1064 and 532 nm on both sides. To efficiently dissipate the heat deposition, the laser crystal and the KTP are wrapped with indium foil and fitted into water cooled copper holder, maintaining at a constant low temperature of 14 °C during the experiment. A Z-type folded cavity consists of four mirrors. The lengths of the three cavity arms, L1, L2, and L3, are 570, 760 and 88 mm, respectively. The whole length of the folded cavity is approximately 141.8 cm, corresponding to a roundtrip transit time of ~9.46 ns. Flat mirror M1, AR-coated at 808 nm on its pump side surface and high-reflection (HR)-coated at 1064 and 532 nm on the other surface, is adopted as the input mirror. Concave mirror M2 with a radius of curvature (ROC) of 500 mm and flat mirror M4, acting as the resonator mirrors, are both HR-coated at 1064 and 532 nm. The output mirror M3 with a ROC of 100 mm is HR-coated at 1064 nm and AR-coated at 532 nm and the transmission for the green laser output is about 99.5%. The active Q-switcher is an EOM (BBO crystal, the repetition rate is from 1 kHz to 5 kHz) with a polarizer and a λ/4 plate, while the passive saturable absorbers are a GaAs wafer and a Bi-doped GaAs with a small signal transmission of 92.6% and 77.8% at 1.0 μm, respectively. Using ion bombardment at 500 KeV with a dose of 1 × 10¹⁴ ions/cm², bismuth (Bi) is incorporated in GaAs. Subsequent anneal is obtained in a rapid thermal annealing system in nitrogen ambient. The sample is annealed at the temperature of 700 °С while the annealing duration is set to be 60 s. Anneal conducted in this way can appropriately activate implanted bismuth and eliminate lattice damage while minimizing impurity diffusion [21,22 ].

 

Fig. 1 Experimental configuration for the composite Nd:YVO₄/KTP green lasers. SA: GaAs or Bi-GaAs.

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3. Experimental results and discussions

In the doubly QML multi-segment composite Nd:YVO₄/Nd:YVO₄/Nd:YVO₄/KTP green lasers with EO and Bi-GaAs (GaAs), two different operating statuses are generated during the experimental process: the QML status and the low repetition rate sub-nanosecond status. In the QML status, the repetition rate of the Q-switched envelope is under the control of the EOM while the repetition rate of the mode-locking pulses underneath a Q-switching envelope is owing to the cavity roundtrip transmit time. The shorter pulse duration of the Q-switching envelope is, the fewer mode-locking pulses appear. When the pulse duration of the Q-switching envelope is short enough compared to the cavity roundtrip transmit time, single mode-locking sub-nanosecond pulse underneath a Q-switching envelope can be achieved. Figure 2 shows the mode-locking pulse shapes change process of EOM and Bi-GaAs dual-loss-modulated green laser under a Q-switching envelope with the variation of pump power at 1 kHz repetition rate. The pulse characteristics are recorded by a digital oscilloscope (1 GHz bandwidth and 20 G samples/s sampling rate, Tektronix Inc., USA) and a fast pin photodiode detector with a rise time of 0.4 ns. The pulse duration of the Q-switching envelope tends to decrease with the increase of the pump power, which corresponds to the reduction of the number of mode-locked pulses underneath a Q-switching envelope. From Figs. 2(a)–2(d), there are five, two, one, and one mode-locked pulses underneath a Q-switching envelope, corresponding to 4.35, 6.08, 7.77, and 11.13 W incident pump power, respectively. The pulse train with 1 kHz sub-nanosecond laser at 11.13 W incident pump power is described in Fig. 2(e) and the corresponding data are recorded for pulse stability calculation. To demonstrate the stability of dual-loss-modulated QML sub-nanosecond pulse green laser, the pulse-to-pulse amplitude fluctuation factor is given as the ratio between the largest deviation and the mean pulse amplitude. Hence, the pulse-to-pulse amplitude fluctuation of the 1 kHz sub-nanosecond laser pulses is less than 1%.

 

Fig. 2 Pulse shapes of EOM and Bi-GaAs dual-loss-modulated green laser at different pump powers with 1 kHz repetition rate: (a) 4.35 W; (b) 6.08 W; (c) 7.77 W; (d) and (e) 11.13 W.

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The average output powers versus the incident pump power are recorded in Fig. 3(a) by using a PM100D Energy/Power Meter (Thorlabs Inc., USA). To ensure that no continuous-wave (CW) background is included in the power measurement, CW green laser operation is performed before each QML green laser with Bi-GaAs or GaAs is achieved and the CW output powers are same in the two cases, as shown in Fig. 3(a). One can see that the maximal output power of the CW green laser is 1.83 W under an incident pump power of 11.13 W, corresponding to the optical conversion efficiency of 16.4%. When the EOM and Bi-GaAs are inserted into the cavity, the dual-loss-modulated QML green laser [the solid red symbols in Fig. 3(a)] is realized until the incident pump power reaches 6.93 W and the repetition rate of EOM is fixed at 1 kHz. After that, the laser changes to operate in low repetition rate sub-nanosecond status [the open red symbols in Fig. 3(a)]. While the threshold pump power is 7.77 W for the EOM and GaAs dual-loss-modulated green laser operating at 1 kHz sub-nanosecond regime. Although the small signal transmission of Bi-GaAs is lower than that of GaAs, higher output power is generated by an EOM and Bi-GaAs, compared with that obtained by an EOM and GaAs. At 11.13 W pump power, the maximum average output powers of 0.445 and 0.316 W are obtained by different SAs of Bi-GaAs and GaAs, respectively. Through the inset of Fig. 7 in [23], one can see that the absorption coefficient of Bi-GaAs at 1.2 eV (~1.0 μm) is much higher than that of GaAs. Thus the higher average output power obtained by the EOM and Bi-GaAs may be due to the more efficient absorption of light in Bi-doped GaAs. Though no power saturation is observed during the whole experimental progress, we do not further augment the incident pump power to protect the composite crystal and KTP from damage.

 

Fig. 3 Average output power (a) and pulse duration (b) versus incident pump power.

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The pulse durations of the Q-switching envelope as a relationship of incident pump power with EO and different SAs are depicted in Fig. 3(b). At first, the green lasers operate at the dual-loss-modulated QML regime [solid symbols in Fig. 3(b)], with the increase of incident pump power the lasers run at the low repetition rate sub-nanosecond status [open symbols in Fig. 3(b)]. As shown in Fig. 3(b), shorter pulse duration of the Q-switched envelope is achieved by the doubly modulated EOM and Bi-GaAs green laser. The minimum pulse durations of 540 and 590 ps are obtained by the different SAs of Bi-GaAs and GaAs, respectively. The corresponding pulse shapes are shown in Fig. 4 . However, because this kind of mode-locked pulse underneath a Q-switching envelope generally runs in the sub-nanosecond range, the pulse duration cannot be directly measured owing to the lack of an antocorrelator at 0.53 μm. To effectively estimate the pulse duration of mode-locking pulses, the deployed oscilloscope traces of the mode-locking pulse are adopted, then the pulse duration can be approximately given by the formula $t_{real}=\sqrt{t_{meas}^2-t_p^2-t_{osc}^2}$ [14,15 ], where $t_{real}^{}$ is the real rise time, $t_{meas}^{}$ is the measured average rise time from 10% to 90%, $t_p^{}$ is the responding time of detection device, $t_{osc}^{}$ is the rise time of the oscilloscope [24]. Here the rise time of the oscilloscope is given as $t_{osc}^{}\times BW=0.35\sim 0.4$ (0.35 is used in the following calculation and BW is 1 GHz), and the responding time of detection device is 0.4 ns. The measured average rise times are 620 and 640 ps for the shortest pulse durations achieved by Bi-GaAs and GaAs, respectively. The real rise time can be calculated by the formula given above. Considering the symmetric shape of the mode-locked pulse and according to the definition of the rise time, we can assume the mode-locked pulse duration is approximately 1.25 times more than the real rise time [14,15 ]. Thus, the shortest mode-locked pulse durations with Bi-GaAs and GaAs are 399 and 446 ps, respectively.

 

Fig. 4 Pulse shapes for different SAs: (a) EO + Bi-GaAs; (b) EO + GaAs.

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The time interval of mode-locking pulses underneath a Q-switching envelope agrees with the cavity round-trip time $2L^{\text{'}}/c$, where $L^{\text{'}}$ is the physical length of the cavity and $c$ is the speed of light in vacuum. Incorporated with the pulse duration of a Q-switching envelope, the number of mode-locked pulses can be calculated. Then the single pulse energy of the two statuses can be calculated, which is shown in Fig. 5 . One can see that larger single pulse energy is obtained by the dual-loss-modulated green laser with EOM and Bi-GaAs. The highest single pulse energies are 445 and 316 μJ with different SAs of Bi-GaAs and GaAs at the incident pump power of 11.13 W and the repetition rate of 1 kHz, respectively. Under the same pump power, the maximal peak powers are 1115 and 709 kW, which are much higher than that achieved in Q-switching or continuous-wave mode-locking laser. The highest peak powers obtained in [16] and [17] were 150 and 560 kW with the pump powers of 8.76 and 18.27 W, respectively. Specially, in [18], the 1022 kW peak power was achieved at 15.3 W pump power, while the peak power was less than 500 kW under 11.13 W pump power. Thus the peak power obtained by EOM and Bi-GaAs is more than twice that in [18] at the pump power of 11.13 W. So the combination of multi-segment composite Nd:YVO₄/Nd:YVO₄/Nd:YVO₄ crystal and Bi-GaAs saturable absorber is a promising candidate for generating high peak power sub-nanosecond pulse.

 

Fig. 5 Single pulse energy of two statuses versus incident pump power.

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4. Conclusions

In conclusion, the first investigation of dual-loss-modulated multi-segment composite Nd:YVO₄/Nd:YVO₄/Nd:YVO₄/KTP green lasers with EO and different SAs (Bi-GaAs and GaAs) has been presented with sub-nanosecond pulse duration at 1 kHz repetition rate. At an incident pump power of 11.13 W, the maximal average output power and the shortest pulse duration of doubly modulated green laser with EO and Bi-GaAs are 0.445 W and 399 ps, which correspond to 445 μJ single pulse energy and 1.115 MW peak power. The doubly EO and Bi-GaAs modulated technique in QML green laser is demonstrated as an efficient means to generate low repetition rate sub-nanosecond pulse with high peak power. The experimental results indicate that bismuth doped GaAs is a promising sasturable absorber for solid-state laser.