1064/1319 nm Dual-Wavelength Alternating Electro-Optic Q-Switched Laser Based on the Common Q-Switching Bias Voltage

: A dual-wavelength alternating electro-optic (EO) Q-switched laser operating at 1064 and 1319 nm is designed, which takes the structure of double the gain crystals and a single EO modulator with the common Q-switching bias voltage (CQBV). The output characteristics of alternating dual-wavelength pulse lasers are studied via simulations and experiments. The results show that the energy ratio of the two lasing wavelengths can be controlled by changing the CQBV. This is because the CQBV affects the loss of two resonators, 1064 and 1319 nm, at the same time. The gain–loss relationship in the dual-wavelength laser resonators can be controlled by changing the CQBV in a certain range.


Introduction
Compared with the dual-wavelength pulse laser achieved by acoustic-optic or passive Q-switched technology, the dual-wavelength pulse laser obtained by EO Q-switched technology has the virtues of high peak power, high stability and a narrower pulse width.Based on this, EO Q-switched dual-wavelength lasers have wide applications in nonlinear frequency conversion, laser drilling, laser spectroscopy, laser ranging and other fields [1][2][3][4][5].However, at present, the dual-wavelength pulse lasers obtained by EO Qswitched technology are output at the same time, which greatly limits its applications [6][7][8].For example, dual-wavelength pulse lasers with a time delay are required to act on metal samples one after another in laser-induced breakdown spectroscopy (LIBS).Since LIBS using a dual-wavelength laser emitted at the same time cannot separate the two processes of laser stripping and plasma induction, the sensitivity of sample analysis is low.Once a certain element has a relatively low content in the sample to be tested, it cannot be detected.However, for the dual-wavelength alternating pulse lasers, the first pulse is used to peel off the sample, and the high-temperature plasma is excited.When the plasma plume begins to cool down, the second pulse is used as the excitation light source of the sample.At the same time, the plasma generated by the first pulse is also re-excited.The dual-wavelength pulse lasers are separated in the time domain.As a result, the two processes of laser stripping and the subsequent excitation of the sample are separated in time [9][10][11].The atomic emission spectrum is enhanced.The sensitivity of spectral analysis is improved [12,13].That is to say, compared with LIBS using a dual-wavelength laser emitted at the same time, LIBS using a dual-wavelength laser with a time delay has a lower detection limit for elements.As a result, elements with a relatively low content in the sample are also reported [14,15].However, to improve the spatial resolution and sensitivity of sample detection, it is essential to control the energy ratio of the alternating dual-wavelength pulse lasers [16].
There are several ways to control the output energy ratio of two pulse lasers, such as controlling the energy ratio by setting the transmittance of the two lasing wavelengths at the output mirror.The disadvantage of this method is that it is difficult to get the best coating results because of the influence of the coating accuracy at home and abroad.In addition, due to the long coating period, it is difficult to change once the coating parameters are determined, which is not conducive to the rapid adjustment of laser experiments.Then, the energy ratio can also be controlled by changing the resonator length of the two lasing wavelengths.However, it is hard to reach the accuracy of the theoretical calculation by manually adjusting the length of the resonator.Alternatively, the energy ratio can also be controlled by changing the voltage of EO modulator.
In 2015, Men S. et al. achieved 1047 nm and 1053 nm dual-wavelength pulse lasers output using double Nd:YLF crystals pumped by a xenon lamp and a single EO modulator.While the λ/4 high voltage was loaded on the EO crystal, the maximum energies of 1047 nm and 1053 nm are 66.2 mJ and 83.9 mJ.The two laser pulse widths were both 18 ns [17].In 2021, Wang C. et al. obtained 1064 and 1342 nm dual-wavelength lasers output by utilizing the structure of a single laser crystal combined with a double RbTiOPO 4 (RTP) EO modulator.The dual-wavelength pulse lasers output with a power ratio of 1:1 was realized by adjusting the voltages of two Q-switched crystals.When the voltage was 2000 V for 1064 nm and 1920 V for 1342 nm, respectively, the dual-wavelength pulse lasers with 1.6 W output power were obtained at 10 kHz repetition and 30 W injection power.The pulse widths of 1064 and 1342 nm were 42 and 81 ns, respectively [18].It can be concluded that the output energy ratio can be controlled by changing the voltage loaded on the shared EO modulator for the dual-wavelength lasers with a small wavelength difference.Since the wavelength difference is small, there is a similar quarter-wavelength Q-switching voltage for the dual-wavelength lasers.The energy ratio can be adjusted between the two optimal crystal voltages.However, there is a great distinction between the quarter-wavelength Q-switching voltage and the stimulated-emission cross-section for the two wavelengths with a large wavelength difference.It is very difficult to choose a common voltage that can not only Q-switch the dual-wavelength lasers at the same time but also adjust the output energy ratio of the two wavelength components.Therefore, only two EO modulators can be selected to correspondingly Q-switch the dual-wavelength lasers.The energy ratio is controlled by adjusting the two Q-switching voltages on the double Q-switched crystals.In addition, if the dual-wavelength lasers use double Q-switches for Q-switching, a single laser crystal is generally used to emit the dual-wavelength lasers.At this time, there is a serious gain competition between the two wavelengths.
In this study, a dual-wavelength alternating EO Q-switched laser is established.The structure of a single Q-switcher with the CQBV shared by two laser gain mediums is used.The dual-wavelength lasers are emitted by two gain mediums.In addition, there is no gain line competition between them.Adjusting the CQBV in a certain range can not only control the energy ratio of the alternating dual-wavelength pulses but also alternately Q-switch the two wavelength components.In addition, the alternating dual-wavelength pulses have a time delay in the time domain.The time interval is adjustable.The results can provide a reliable illumination source for the field of LIBS.

Theoretical Simulation and Analysis
To understand the alternate emission process of 1064 and 1319 nm lasers, a model of a dual-wavelength alternating EO Q-switched laser based on the CQBV is established as follows [19]: where n 1 and n 2 are the inverse populations in the upper level of the 1319 and 1064 nm lasers.∅ 1 and ∅ 2 are the laser photon density of the 1319 nm and 1064 nm lasers, respectively.R p1 and R p2 are the pumping rates for 1319 nm and 1064 nm, respectively.c is the speed of light.σ 1 and σ 2 are the stimulated-emission cross-sections for the 1319 and 1064 nm lasers, respectively.γ is the inversion factor.∆t represents the time delay between the two alternating components.∆τ 1 and ∆τ 2 represent the pulse widths for 1319 nm and 1064 nm, respectively.T represents the EO Q-switched period.τ f is the lifetime of the upper level of Nd:YAG.In the 1319 and 1064 nm laser cavities, the photon decay time τ ci is shown as where l c1 and l c2 are the lengths of the 1319 and 1064 nm laser cavities, respectively.δ 1 and δ 2 are the total losses of the 1319 and 1064 nm lasers, respectively.T 0 is the dissipative loss.R 1 and R 2 are the reflectivities of the 1319 and 1064 nm lasers, respectively, at the output coupler.−ln(R 1 ) and −ln(R 2 ) are the coupling losses of the 1319 and 1064 nm lasers, respectively.L Q1 (t) and L Q2 (t) represent the Q-switched losses for the 1319 and 1064 nm laser cavities, respectively.V 1319/4 and V 1064/4 are the λ/4 voltages for 1319 and 1064 nm, with values of 1500 V and 1200 V, respectively.V (t) is the CQBV applied on or removed from the shared Q-switcher, which changes with time.
Whether the CQBV can alternately Q-switch for two wavelengths depends on the relationship between the total loss and gain in the 1319 and 1064 nm laser resonators.Then, the gain of the two laser resonators can be calculated according to Equation (10) [20].
where G 1 and G 2 are the gain of the 1319 and 1064 nm lasers, respectively.ε 1 and ε 2 are the loss terms of the 1319 and 1064 nm lasers, respectively.R 11 and R 12 are the reflectivities of the 1319 and 1064 nm lasers at the total reflector mirror, respectively.n 01 and n 02 are the initial inverse populations, which can be calculated according to Equation (11).t 0 is the Q-switched time.l is the length of the Nd:YAG rod.
For the dual-wavelength alternating EO Q-switched laser, two laser gain mediums are used to emit dual-wavelength lasers.In addition, the 1064 and 1319 nm lasers share the single EO modulator with the CQBV.The CQBV is used to alternately Q-switch the dual-wavelength lasers.The 1319 nm laser adopts a voltage-increased EO Q-switch by using the rising edge of the CQBV, while the 1064 nm laser adopts a voltage-decreased EO Q-switch by using the falling edge of the CQBV.Therefore, the temporal interval between the two wavelength components is 200 us, corresponding to the duration of the Q-switching bias voltage.If the CQBV can completely and alternately Q-switch the two lasers, it should ensure two points.On the one hand, when the CQBV is applied, a 1319 nm pulse laser is obtained.However, the high voltage lasts 200 µs.In addition, the pumping source of 1064 nm is also working in the 200 µs maintained by the high voltage.Therefore, not only does the gain of the 1319 nm resonator need to be greater than the total loss, but the gain of the 1064 nm resonator also needs to be less than the total loss when the CQBV is applied.In this way, only the 1319 nm pulse laser is output when the common Q-switching voltage is applied.That is to say, the shared EO modulator should turn on 1319 nm and turn off 1064 nm at the same time.On the other hand, when the CQBV is removed (the CQBV is equal to 0), the 1064 nm pulse laser is obtained.However, within the time when the CQBV is 0, the pumping source of the 1319 nm laser is also working at the same time.Therefore, not only does the gain of the 1064 nm resonator need to be greater than the total loss but also the gain of the 1319 nm resonator needs to be less than the total loss when the CQBV is removed.In this way, only the 1064 nm pulse laser is output when the common Q-switching voltage is removed.That is to say, the shared EO modulator should turn on 1064 nm and turn off 1319 nm at this time.Based on these two aspects, Figure 1 shows the numerical simulation of the relationship between the CQBV and the gain/loss in the two resonators.The parameters used in the theoretical calculation are given in Table 1 [21][22][23].
When the CQBV is applied, the shared EO modulator turns on the 1319 nm laser when the CQBV is in the range of 819-2181 V.When the CQBV is in the range of 877-1524 V, the shared EO modulator turns the 1064 nm laser off.In addition, it can be seen from Figure 1 that when the CQBV is removed (meaning the CQBV is equal to 0), the 1319 nm laser gain is less than the total loss.In addition, the 1064 nm total loss is less than the gain.Therefore, the CQBV that can alternately Q-switch dual-wavelength lasers ranges from 877 V to 1524 V.Then, the rate equation of the dual-wavelength alternating EO Q-switched laser is numerically solved by making use of the fourth-order Runge-Kutta method, with the CQBV shared by the dual-wavelength laser as the boundary condition.The photon number density versus time when the CQBV is at 899 V, 1200 V, 1453 V and 1500 V is shown in Figure 2.
Photonics 2023, 10, x FOR PEER REVIEW 4 of 12 using the rising edge of the CQBV, while the 1064 nm laser adopts a voltage-decreased EO Q-switch by using the falling edge of the CQBV.Therefore, the temporal interval between the two wavelength components is 200 us, corresponding to the duration of the Qswitching bias voltage.If the CQBV can completely and alternately Q-switch the two lasers, it should ensure two points.On the one hand, when the CQBV is applied, a 1319 nm pulse laser is obtained.However, the high voltage lasts 200 µs.In addition, the pumping source of 1064 nm is also working in the 200 µs maintained by the high voltage.Therefore, not only does the gain of the 1319 nm resonator need to be greater than the total loss, but the gain of the 1064 nm resonator also needs to be less than the total loss when the CQBV is applied.In this way, only the 1319 nm pulse laser is output when the common Q-switching voltage is applied.That is to say, the shared EO modulator should turn on 1319 nm and turn off 1064 nm at the same time.On the other hand, when the CQBV is removed (the CQBV is equal to 0), the 1064 nm pulse laser is obtained.However, within the time when the CQBV is 0, the pumping source of the 1319 nm laser is also working at the same time.Therefore, not only does the gain of the 1064 nm resonator need to be greater than the total loss but also the gain of the 1319 nm resonator needs to be less than the total loss when the CQBV is removed.In this way, only the 1064 nm pulse laser is output when the common Q-switching voltage is removed.That is to say, the shared EO modulator should turn on 1064 nm and turn off 1319 nm at this time.Based on these two aspects, Figure 1 shows the numerical simulation of the relationship between the CQBV and the gain/loss in the two resonators.The parameters used in the theoretical calculation are given in Table 1 [21][22][23].When the CQBV is applied, the shared EO modulator turns on the 1319 nm laser when the CQBV is in the range of 819 V-2181 V. When the CQBV is in the range of 877 V-1524 V, the shared EO modulator turns the 1064 nm laser off.In addition, it can be seen from Figure 1 that when the CQBV is removed (meaning the CQBV is equal to 0), the 1319 nm laser gain is less than the total loss.In addition, the 1064 nm total loss is less than the gain.Therefore, the CQBV that can alternately Q-switch dual-wavelength lasers ranges from 877 V to 1524 V.Then, the rate equation of the dual-wavelength alternating EO Qswitched laser is numerically solved by making use of the fourth-order Runge-Kutta method, with the CQBV shared by the dual-wavelength laser as the boundary condition.The photon number density versus time when the CQBV is at 899 V, 1200 V, 1453 V and 1500 V is shown in Figure 2. As shown in Figure 2, the peak photon number density of the 1319 nm laser is positively correlated with the CQBV.The peak photon number density of the 1064 nm laser increases at first and then decreases with the increase in the CQBV.When the CQBV is 1200 V, the peak photon number density of the 1064 nm laser reaches the maximum.In addition, when the CQBV is less than 1453 V, the peak photon density ratio of the 1319/1064 nm laser is less than 1.When the CQBV is 1500 V, the peak photon density ratio of the 1319/1064 nm laser is greater than 1.The simulation result shows that the photon number density ratio of the two wavelengths of the 1319/1064 nm laser reaches 1:1 when the CQBV ranges from 1453 V to 1500 V.This provides an effective way to realize the 1:1 output energy ratio of the dual-wavelength alternating Q-switched laser based on the CQBV.As shown in Figure 2, the peak photon number density of the 1319 nm laser is positively correlated with the CQBV.The peak photon number density of the 1064 nm laser increases at first and then decreases with the increase in the CQBV.When the CQBV is 1200 V, the peak photon number density of the 1064 nm laser reaches the maximum.In addition, when the CQBV is less than 1453 V, the peak photon density ratio of the 1319/1064 nm laser is less than 1.When the CQBV is 1500 V, the peak photon density ratio of the 1319/1064 nm laser is greater than 1.The simulation result shows that the photon number density ratio of the two wavelengths of the 1319/1064 nm laser reaches 1:1 when the CQBV ranges from 1453 V to 1500 V.This provides an effective way to realize the 1:1 output energy ratio of the dual-wavelength alternating Q-switched laser based on the CQBV.

Experimental Setup
The experimental setup of the 1064 and 1319 nm dual-wavelength alternating EO Qswitched lasers based on the CQBV is shown in Figure 3.An alternating output for the 1064 and 1319 nm dual-wavelength pulse lasers is achieved by making use of the double-cavity structure of, the double laser crystals and a single EO Q-switcher.The linear resonator provides the optical feedback and laser output for 1319 nm, whereas the folded cavity provides the optical feedback and laser output for 1064 nm.A single EO modulator with the CQBV and an output mirror are shared by two resonators for both components.The linear resonator, with a length of 26.9 cm, is made up of M1, an Nd : YAG (for 1319 nm) rod, P1, C1, a Q-switcher and M5.The folded cavity, with a length of 30 cm, is made up of M3, an Nd : YAG (for 1064 nm) rod, P2, M4, M2, a Q-switcher and M5.M1 is the planoconcave total reflective mirror with the coated parameters of high reflectivity (HR)@1319 nm (R > 99%) and high transmission (HT)@1064 nm (T > 99%) on the concave side.M3 is the plano-concave total reflective mirror with the coated parameter of HR@1064 nm on the concave side (R > 99%).The doping concentration and the size are the same for Nd : YAG (for 1064 nm) and Nd : YAG (for 1319 nm).The concentration of Nd 3+ in Nd:YAG is 0.6 at.%.The crystal size is φ4 × 80 mm.The HT coatings for 1064 and 1319 nm are plated on the ends (R < 0.5%).Two Nd:YAG rods are unilateral-side-pumped by two 808 nm laser diode (LD) stacked arrays.The pumping light passes through a U-shaped condenser cavity into an Nd:YAG crystal.The LD stacked array contains four stacked blocks.In addition, each stacked block is composed of 10 single bars.The stacked array pumping power is 3600 W. The TEC temperature controller is used to keep two LD stacked arrays at a constant temperature of 323 K.The two LD stacked arrays are powered by two laser power supplies.P1 and P2 are polarizers.C1 is a quarter-wave plate.The coated parameters of 45 • mirror M2 are that the left surface S1 is plated with HT@1319 nm and the right surface S2 is plated with HT@1319 nm and HR@1064 nm.Moreover, M4 is a 45 • mirror with the coated parameter of HR@1064 nm (R > 99%).M5 is the output mirror with a double-sided coated.The parameter of the left surface S1 is anti-refletance@1064 + 1319 nm (T > 99%), and the right surface S2 is coated with dichroic film with transmittance of 75% for 1064 nm and transmittance of 20% for 1319 nm.In order to compensate for the natural birefringence effect of the RTP crystal caused by temperature, two identical orthogonal RTP crystals are employed as the shared EO modulator [24,25].The dimensions of the two x-cut RTP crystals are 6 × 6 × 10 mm 3 .The films of HT@(1064 + 1319)nm are coated on both end faces of the RTP crystals.The two RTP crystals are parallel to each other along the z-axis (the direction of light passing).Furthermore, the x-axis of one of the two RTP crystals is parallel to the y-axis of the other.That is, the two RTP crystals rotate 90 • relative to each other.The two x-z faces perpendicular to the y-axis of each RTP modulator are the electrodes of the common EO modulator.The three-dimensional diagram of the shared EO modulator is shown in Figure 4.Moreover, the alternating output of the 1064 and 1319 nm pulses depends not only on the structure design, as shown in Figure 3, but also on the timing relationship of the driving signals among the RTP crystals, the pumping source for 1064 nm and the pumping source for 1319 nm .The timing control relationship of the three driving signals is shown in Figure 5.The two Nd:YAG rods are alternately pumped by two LD stacked arrays on the unilateral side.Every gain media is pumped by the pumping source with a 100 ms interval and a 200 µs pumping duration.The pumping source of the 1319 nm laser starts pumping at first.From the structure shown in Figure 3, it can be seen that the voltage-increased EO Q-switched technology is adopted by 1319 nm and the voltage-decreased EO Qswitched technology is utilized by 1064 nm.Therefore, the 1319 nm resonator is under Moreover, the alternating output of the 1064 and 1319 nm pulses depends not only on the structure design, as shown in Figure 3, but also on the timing relationship of the driving signals among the RTP crystals, the pumping source for 1064 nm and the pumping source for 1319 nm.The timing control relationship of the three driving signals is shown in Figure 5.Moreover, the alternating output of the 1064 and 1319 nm pulses depends not only on the structure design, as shown in Figure 3, but also on the timing relationship of the driving signals among the RTP crystals, the pumping source for 1064 nm and the pumping source for 1319 nm .The timing control relationship of the three driving signals is shown in Figure 5.The two Nd:YAG rods are alternately pumped by two LD stacked arrays on the unilateral side.Every gain media is pumped by the pumping source with a 100 ms interval and a 200 µs pumping duration.The pumping source of the 1319 nm laser starts pumping at first.From the structure shown in Figure 3, it can be seen that the voltage-increased EO Q-switched technology is adopted by 1319 nm and the voltage-decreased EO Qswitched technology is utilized by 1064 nm.Therefore, the 1319 nm resonator is under The two Nd:YAG rods are alternately pumped by two LD stacked arrays on the unilateral side.Every gain media is pumped by the pumping source with a 100 ms interval and a 200 µs pumping duration.The pumping source of the 1319 nm laser starts pumping at first.From the structure shown in Figure 3, it can be seen that the voltage-increased EO Q-switched technology is adopted by 1319 nm and the voltage-decreased EO Q-switched technology is utilized by 1064 nm.Therefore, the 1319 nm resonator is under high loss when the shared Q-switcher is not loaded with the CQBV in 0-200 µs.Then, the 1319 nm pulse is obtained by loading the CQBV at 200 µs.The gain medium of 1064 nm is under population inversion when the shared EO modulator is loaded with the CQBV in 200-400 µs.When the CQBV is removed from the shared EO modulator at 400 µs, the 1064 nm pulse laser lagged behind the 1319 nm pulse laser at 200 µs in the time domain that is achieved.In addition, the dual-wavelength alternating EO Q-switched laser is equipped with tuning software.It is mainly used to control the timing sequence relationship among the driving signals of the 1064 and 1319 nm laser power supplies and the EO crystal driver module.By adjusting the trigger time, relative delay time and duration of the three signals, the temporal interval between the two wavelength components can be tuned.
During the experiment, the CQBV applied to the shared Q-switcher can be adjusted by the EO crystal driver module to gain the alternating output of the 1064 and 1319 nm dual-wavelength pulse lasers.

Experimental Results and Discussion
After collimating the optical path, the working frequency of the laser is set to 10 Hz.According to the numerical simulation result, the alternate Q-switching range of the CQBV is from 877 V to 1524 V. Therefore, the CQBV applied to the shared EO modulator is selected to be 1100 V. First, the output spectrum is recorded by using a spectrometer (AvaSpec-2048-USB2, Avantes) under the injection energy of 260 mJ for 1064 nm and the injection energy of 665 mJ for 1319 nm.The wavelengths of the two pulse lasers are 1064 nm and 1319 nm, as illustrated in Figure 6.Then, the two lasing components are separated outside the cavity by using a spectroscope.Under the same repetition frequency, CQBV and injection pumping energy conditions, the sequences of the dual-wavelength lasers are measured by using the method of connecting two photodetectors (DET 10 D/M, DET 10 A/M, Thorlabs) to two channels of the same oscilloscope (DPO7104C, Tektronix). Figure 7a shows that both 1064 and 1319 nm are output at the repetition frequency of 10 Hz.As shown in Figure 7b, the dual-wavelength lasers are separated in the time domain by amplifying any group of dual-wavelength pulsed lasers.The 1064 nm and 1319 nm are alternately output.The 1319 nm pulse takes the lead in output, while the 1064 nm pulse lags behind the 1319 nm pulse at 200 µs.
by the EO crystal driver module to gain the alternating output of the 1064 and 1319 nm dual-wavelength pulse lasers.

Experimental Results and Discussion
After collimating the optical path, the working frequency of the laser is set to 10 Hz.According to the numerical simulation result, the alternate Q-switching range of the CQBV is from 877 V to 1524 V. Therefore, the CQBV applied to the shared EO modulator is selected to be 1100 V. First, the output spectrum is recorded by using a spectrometer (AvaSpec-2048-USB2, Avantes) under the injection energy of 260 mJ for 1064 nm and the injection energy of 665 mJ for 1319 nm.The wavelengths of the two pulse lasers are 1064 nm and 1319 nm, as illustrated in Figure 6.Then, the two lasing components are separated outside the cavity by using a spectroscope.Under the same repetition frequency, CQBV and injection pumping energy conditions, the sequences of the dual-wavelength lasers are measured by using the method of connecting two photodetectors (DET 10 D/M, DET 10 A/M, Thorlabs) to two channels of the same oscilloscope (DPO7104C, Tektronix). Figure 7a shows that both 1064 and 1319 nm are output at the repetition frequency of 10 Hz.As shown in Figure 7b, the dual-wavelength lasers are separated in the time domain by amplifying any group of dual-wavelength pulsed lasers.The 1064 nm and 1319 nm are alternately output.The 1319 nm pulse takes the lead in output, while the 1064 nm pulse lags behind the 1319 nm pulse at 200 µs.Finally, when the injection energies of the 1064 and 1319 nm lasers are 260 mJ and 665 mJ, respectively, the output characteristics of dual-wavelength alternating EO Qswitched laser are recorded when the CQBV is at different voltages.As shown in Figure 8, the range of the CQBV that can alternately turn off the Q-switch for two lasers is 829 V to 1487 V.Although the Q-switching length is 658 V, the different common Q-switching Finally, when the injection energies of the 1064 and 1319 nm lasers are 260 mJ and 665 mJ, respectively, the output characteristics of dual-wavelength alternating EO Qswitched laser are recorded when the CQBV is at different voltages.As shown in Figure 8, the range of the CQBV that can alternately turn off the Q-switch for two lasers is 829 V to 1487 V.Although the Q-switching length is 658 V, the different common Q-switching bias voltages correspond to the different output characteristics of the two pulse lasers.The energy of the 1064 nm pulse increases at first and then decreases with the increase in the CQBV.The energy of the 1064 nm pulse reaches the peak when the CQBV is 1200 V.In addition, the maximum energy is 43 mJ.The energy of the 1319 nm pulse is positively correlated with the CQBV.The energy of the 1319 nm pulse reaches the peak when the CQBV is 1487 V.Moreover, the maximum energy is 40.1 mJ.It can be seen that the energy of the two lasers varies with the CQBV.This is because when the CQBV changes, the Q-switching loss in the two resonators also changes, which affects the dual-wavelength laser output characteristics.The wavelength difference (255 nm) between 1064 nm and 1319 nm is large.The stimulated-emission cross-section (5:1) and the quarter-wavelength voltage (1200 V@1064 nm, 1500 V@1319 nm) are also different.Despite the above differences between the two lasing components, a range of the CQBV can still be found that can alternately Q-switch the dual-wavelength lasers.In addition, the loss in the two resonators can be controlled by optimizing the CQBV to control the output characteristics of the two lasing components.When the CQBV is 1487 V, the output energy of 1064 nm is 40.13 mJ, which is approximate to the output single pulse energy of 40.1 mJ for 1319 nm.In addition, a 1:1 ratio of the output single pulse energy is obtained.When the output energies are 40.13 and 40.1 mJ, the pulse widths of 1064 and 1319 nm are 13.62 and 14.57 ns, respectively, as shown in Figure 9. Figure 10 shows the stability of the energy of the two wavelengths.The energy stabilities of the 1064 and 1319 nm lasers are 0.1% and 0.76%, respectively.

Conclusions
In summary, an all-solid-state Nd : YAG laser with alternating dual wavelengths of 1064 and 1319 nm is designed.A structure with double the gain crystals and a single common EO modulator with the CQBV is built.A rate equation model of the dualwavelength alternating EO Q-switched laser based on the CQBV is established.It is theoretically and experimentally proven that the output energy ratio of the dual-wavelength alternating pulse lasers can be controlled by adjusting the CQBV.At a repetition frequency of 10 Hz, the range of the CQBV that can alternately Q-switch for two lasers is 829 V to 1487 V when the injection energy is 260 and 665 mJ for 1064 and 1319 nm, respectively.The output of the alternating dual-wavelength pulse lasers with an energy ratio of 1:1 is obtained when the CQBV is 1487 V.The output energies of 1064 and 1319 nm are 40.13mJ and 40.1 mJ, respectively.At this time, the pulse widths of 1064 and 1319 nm are 13.62 and 14.57 ns, respectively.The results provide a reliable illumination source for the field of LIBS.

Figure 1 .
Figure 1.The relationship between gain/loss and the CQBV in 1064 and 1319 nm laser resonators.

Figure 1 .
Figure 1.The relationship between gain/loss and the CQBV in 1064 and 1319 nm laser resonators.

Figure 2 .
Figure 2. The photon number density versus time when the CQBV is at (taking the first pulse of the dual−wavelength laser as a reference) (a) 899 V; (b) 1200 V; (c) 1453 V; (d) 1500 V.

Figure 2 .
Figure 2. The photon number density versus time when the CQBV is at (taking the first pulse of the dual−wavelength laser as a reference) (a) 899 V; (b) 1200 V; (c) 1453 V; (d) 1500 V.

Figure 4 .
Figure 4.The three−dimensional diagram of the shared EO modulator.

Figure 5 .
Figure 5.The temporal relation of driving signals among RTP modulator and pumping sources for 1064 and 1319 nm.

Figure 4 .
Figure 4.The three−dimensional diagram of the shared EO modulator.

Figure 4 .
Figure 4.The three−dimensional diagram of the shared EO modulator.

Figure 5 .
Figure 5.The temporal relation of driving signals among RTP modulator and pumping sources for 1064 and 1319 nm.

Figure 5 .
Figure 5.The temporal relation of driving signals among RTP modulator and pumping sources for 1064 and 1319 nm.

Figure 6 .
Figure 6.The spectrogram of the dual-wavelength alternating EO Q-switched laser.

Figure 8 .
Figure 8. Schematic diagram of 1064 and 1319 nm output energy when the CQBV is adjusted.

Figure 9 .
Figure 9. Diagrams of 1064 and 1319 nm dual-wavelength laser pulse widths.(a) the pulse width of 1319 nm; (b) the pulse width of 1064 nm.

Figure 8 .
Figure 8. Schematic diagram of 1064 and 1319 nm output energy when the CQBV is adjusted.The existing dual-wavelength EO Q-switched lasers can only use one of the voltageincreased EO Q-switched technology and the voltage-decreased EO Q-switched technology.The dual-wavelength pulse laser is output at the same time, which cannot be actively regulated.In other words, the time delay between the two signals is zero.Compared with the existing dual wavelength EO Q-switched lasers, the dual wavelength laser with common Q-switching voltage proposed in this paper skillfully combines voltage-increased and voltage-decreased EO Q-switched technologies.The dual wavelength pulse laser is separated in the time domain and is alternately output.Moreover, the time delay between the two signals can be adjusted.

Figure 8 .
Figure 8. Schematic diagram of 1064 and 1319 nm output energy when the CQBV is adjusted.

Figure 9 .
Figure 9. Diagrams of 1064 and 1319 nm dual-wavelength laser pulse widths.(a) the pulse width of 1319 nm; (b) the pulse width of 1064 nm.

Figure 10 .
Figure 10.The stability of energy of 1064 and 1319 nm.

Figure 9 .
Figure 9. Diagrams of 1064 and 1319 nm dual-wavelength laser pulse widths.(a) the pulse width of 1319 nm; (b) the pulse width of 1064 nm.

Figure 8 .
Figure 8. Schematic diagram of 1064 and 1319 nm output energy when the CQBV is adjusted.

Figure 9 .
Figure 9. Diagrams of 1064 and 1319 nm dual-wavelength laser pulse widths.(a) the pulse width of 1319 nm; (b) the pulse width of 1064 nm.

Figure 10 .
Figure 10.The stability of energy of 1064 and 1319 nm.

Figure 10 .
Figure 10.The stability of energy of 1064 and 1319 nm.

Table 1 .
The parameters of the numerical simulation.