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Abstract
The antiphase dynamics of Q-switched orthogonally polarized emissions have been thoroughly investigated. A Nd:YLF crystal with the anisotropic thermal lensing effect is used as the gain medium for achieving dual polarized laser. By using the Cr4+:YAG saturable absorber, the passively Q-switched output shows intriguing switching dynamics, where the number of pulses for both polarized components within one switching period is directly determined by the power ratio between the orthogonally polarized emissions. Experimental results reveal that the pulse energies of every single pulse for both orthogonally polarized states are equal with the maximum value of 223 μJ. The pulse durations for π- and σ-polarization are measured to be 15 ns and 11 ns and the corresponding peak power levels are up to 15.0 kW and 20.3 kW, respectively.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Laser sources consist of two orthogonally polarized states have received much attention due to their various applications including precision metrology, secure communication and holography [1-4]. To realize the orthogonally polarized lasers, one of the common methods is using a birefringent element or an etalon to control the gain-to-loss balance for achieving the orthogonally polarized operation [5,6]. To avoid the extra internal loss of additional elements, the orthogonally polarized lasers can also be obtained by introducing the stress-induced birefringence in the isotropic gain medium to split the lasing modes into two polarization states [7-9]. Recently, an orthogonally polarized oscillation with separated emission wavelengths has been observed and analyzed in a concave-plano laser cavity without any additional optical component [10-14]. With the spectral and thermal-induced anisotropy of the gain crystals, the power ratio between the orthogonally polarized emissions can be flexibly adjusted by varying the incident pump power.
In comparison with continuous-wave (CW) lasers, laser sources with high pulse energy and high peak power are favorable for numerous applications. To achieve high peak power lasers, passively Q-switched (PQS) operation with a saturable absorber has been widely exploited because of its advantages of low cost, high stability, and compact cavity design. Owing to the good optical and physical stability such as high damage threshold, long lifetime, high thermal conductivity, and excellent optical quality, Cr4+:YAG crystal has served as a promising saturable absorber in the near infrared region for a long time. Based on the theory of passive Q-switching, laser crystal with a long upper-state lifetime is highly beneficial to generate large pulse energy. Both experimental and theoretical results have revealed that the Nd:YLF crystal with a relatively long upper-level lifetime is an excellent gain medium for generating high pulse energy output [15-17]. Recently, a novel concept has been proposed to achieve the orthogonally polarized self-mode-locked Nd:YLF laser by controlling the relative gain of the polarized states via the anisotropic thermal lens effect [12]. Therefore, it is of great interest to extend this concept to explore the Q-switched performances and temporal dynamics of the orthogonal polarized states for the dual-wavelength Nd:YLF laser with orthogonally polarized states.
In this work, temporal dynamics of pulse trains of an orthogonally polarized dual-wavelength PQS Nd:YLF laser is thoroughly studied by exploring the polarization-resolved output emissions. At an incident pump power of 10.4 W, the maximum average output power of 1.55 W is achieved under the optimized PQS operation. The pulse repetition rate is measured to be 6.25 kHz and the pulse energy is calculated to be 248 μJ. Most interestingly, the stable output pulse trains with little amplitude fluctuation can be observed to exhibit complex polarization switching dynamics which was attributed to the gain competition between two polarized states. The ratio of number of pulses between orthogonally polarized emissions within one period of polarization switching can be found to be consistent with the power ratio between the orthogonally polarized states. This good agreement indicates that each polarized output has the same output pulse energy but operates in opposite phase. The pulse durations are measured to be 15 ns and 11 ns for π- and σ-polarization which corresponding to the peak power of 15.0 kW and 20.3 kW, respectively.
2. Experimental setup
The scheme of the PQS orthogonally polarized Nd:YLF laser is depicted in Fig. 1. The resonator was designed to be a linear concave-plano cavity with a cavity length of 61 mm. A 16-W, 806-nm fiber-coupled laser diode with a core diameter of 200 μm was used as the pump source. The pump light was reimaged into the laser crystal by a focusing lens pairs with 50/50-mm focal lengths and a coupling efficiency of 90%. The gain crystal was an a-cut 0.8 at.% Nd:YLF crystal with a length of 20 mm and a square aperture of 3 x 3 mm2. Both surfaces of the gain medium were coated to be anti-reflectance (AR < 0.2%) at 806 nm and 1050 nm. The saturable absorber was a cylindrical Cr4+:YAG crystal with 3 mm in diameter and 2 mm in length. The initial transmission of the Cr4+:YAG crystal was 80% and it was placed near to the flat output coupler. Both gain medium and saturable absorber were wrapped with indium foil and mounted in water-cooled copper holders with a temperature of 20 °C. For good PQS operation, the second threshold condition is given by [18]:
where T0 is the initial transmission of the Cr4+:YAG crystal, R is the reflectivity of the output coupler, L is the nonsaturable round-trip dissipative loss, A/As is the ratio of the mode size in the gain medium to that in the saturable absorber, σgs is the ground-state absorption cross section of the saturable absorber, σ is the stimulated emission cross section of the gain medium, γ is the inversion reduction factor, and β is the ratio of the excited-state absorption cross section to that the ground-state absorption in the saturable absorber. By substituting σπ = 1.8 × 10−19 cm2, σσ = 1.2 × 10−19 cm2, σgs = 4.3 × 10−18 cm2, T0 = 80%, R = 85%, β = 0.2, and γ = 1 into Eq. (1), it can be seen that the criterion directly can be satisfied regardless of the ratio of the mode size A/As. As a consequence, the influence of the radius of curvature (ROC) of the concave mirror on the PQS performance can be ignored. In addition, both experimental and theoretical results have demonstrated that the focal lengths of thermal lens for Nd:YLF crystal along π and σ axes are negative and positive, respectively due to its anisotropic thermo-optical coefficient and thermo-expansion [19,20]. Because of the specifically negative dependence of the refractive index on temperature at π-polarization, there exists a critical power Pcri which the thermal lens will cause the resonator to be unstable and induces a diffraction loss for 1047-nm emission. As a results, it is possible to suppress the 1047-nm emission and achieve orthogonally polarized operation by controlling the pump power and choosing a suitable ROC of input mirror. The input mirrors with ROC of 300 mm was coated with high-transmittance (HT, T > 95%) at 806 nm and with high-reflectance (HR, R > 99.9%) at 1030-1080 nm. A flat mirror with partial reflectance of R = 85% at 1050 nm was utilized as the output coupler. An optical spectrum analyzer with the resolution of 0.1 nm (Advantest, Q8381) was employed to record the spectrum information.3. Experimental results and discussion
Initially, we removed the saturable absorber to investigate the CW performance of the Nd:YLF laser. The average output power versus pump power is presented in Fig. 2. The pump threshold was experimentally found to be 0.5 W. The maximum output power of 3.43 W can be obtained at the pump power of 10.4 W, corresponding to an optical-to-optical conversion efficiency of 33%. The slope efficiency of the laser is determined to be 33.6%. After that, we inserted the Cr4+:YAG crystal into the laser cavity to investigate the PQS performance completely. In the first place, we recorded the output behavior of overall output without separating polarization states. The pump threshold for the PQS operation was 2.0 W and an average output power of 1.55 W was obtained at the pump power of 10.4 W, as shown in Fig. 2. The Q-switching conversion efficiency was found to be 45%. Figure 3(a) shows the pulse energy and overall pulse repetition rate as a function of the pump power. It can be seen that the pulse repetition rate varies linearly from 2.15 kHz to 6.25 kHz as the pump power increases from 3.2 W to 10.4 W. Accordingly, the pulse energy increased with the pump power and it was approximately up to 248 μJ at the pump power of 10.4 W. The pulse temporal behavior was recorded by a digital oscilloscope (LeCroy, WaveMaster 813Zi-B 40Gs/s, 13GHz bandwidth) with a high-speed InGaAs photodetector (Electro-optics Technology, ET-3500 with 10 GHz bandwidth). A typical oscilloscope trace of the total output pulses at pump power of 7.5 W is shown in Fig. 3(b) with a time span of 800μs. The overall pulse-to-pulse amplitude fluctuation was found to be within 5%. From Fig. 3(b), we also observed that there was no additional pulse or satellite pulse in every single Q-switched shot. It revealed the fact that there was no simultaneous emission along orthogonal polarization states.
Fig. 2 Average output powers versus the pump power in continuous-wave and passively Q-switched operations.
Fig. 3 (a) Experimental results for pulse energy and pulse repetition rate versus pump power; (b) typical oscilloscope trace of the output pulse train with a time span of 8 ms at pump power of 7.5 W.
To investigate the polarization state of the output emission, we further measured the output power for the π- and σ-polarized states and total output as a function of pump power. As shown in Fig. 4(a), the PQS laser exhibits linear polarization at low pump power and the polarization state maintained along π-polarization until reaching its maximum output power of 0.67 W. As the pump power increased, the lasing polarization started to switch to the σ-polarization with great slope efficiency. From the standard ABCD matrix approach, the effective focal power of the thermal lens D is given by [12]:
where ρ is the ROC of the concave mirror, d1 and d2 are the optical path length between the mirrors and the incident end of the gain crystal. Considering the thermal lens to be negative and setting the separations d1 ≈0, d2 < ρ, the critical focal power Dcri can be obtained as . Moreover, the effective focal length of end-pumped solid-state laser can be approximately expressed by [21]: where ωp(z) is the variation of the pump radius, ω0 is the beam waist, z0 is a distance from the entrance of the gain crystal, lcry is the length of gain crystal, α is the absorption coefficient at the pump wavelength λp, Kc is the thermal conductivity, ξ is the fractional thermal loading, dn/dT is the temperature dependence of the refractive index, M2 is the pump beam quality factor, n is the refractive index, and Pin is the pump power. From Eqs. (3), the critical pump power can be given by:By substituting the parameters: Kc = 6.3 W/m-K, dn/dT = −4.3 × 10−6 K−1, lcry = 20 mm, α = 0.18 mm−1, and ρ = 300 mm, ξ = 0.28, λp = 806 nm, n = 1.47, M2 = 115, ω0 = 120 μm, z0 = 1.7 mm, the critical pump power is calculated to be 6.1 W. It can be seen that numerical calculation is consistent with the experimental result. Figure 4(b) depicts the lasing spectra for balanced dual-polarization output power at the pump power of 7.5 W. The central wavelengths for two polarization states were located at 1047 nm and 1053 nm corresponding to the two Stark levels transition of Nd:YLF crystal. Moreover, by the knife edge method, the beam quality better than 1.3 was calculated under dual-polarized operation.Fig. 4 (a) Average output power for total output power at π- and σ-polarization states versus pump power; (b) optical spectra for the dual-wavelength PQS laser at a pump power of 7.5 W.
Figure 5 shows the oscilloscope traces of the PQS laser for π- and σ-polarization states at different pump powers. It can be seen that the laser displayed almost linear polarization and stable pulse with uniform amplitude at both lower and higher pump power of 6.0 W and 10 W, as shown in Figs. 5(a) and 5(f), respectively. As soon as the pump power increased to the regime of polarization coexistence, the polarization-resolved pulse trains were observed to exhibit a phenomenon of complex polarization switching dynamics, as shown in Figs. 5(b)-5(e). Consequently, the irregular antiphase dynamics need to be improved via carefully fine tuning of the laser cavity through monitoring the temporal pulse trains. Experimental results reveal that the average output power of the temporal optimization is approximately 90% of the maximum average output power. Figures 6(a)-6(d) show the experimental results for the case of output pulse trains after temporal optimization. It can be seen that the polarization-resolved pulse trains reveal nearly perfect antiphase dynamics. As the pump power was 7.5 W, the two polarized states oscillate with nearly the same repetition rate of 2.1 kHz. In comparison with the total output, each polarized state was shifted within 1/2 period with each other and displayed a classical antiphase dynamics, as shown in Fig. 6(a). As the pump power increased, the output emission along π-polarization state maintained stable pulse trains. On the other hand, the σ-polarization emission exhibited quasi periodic oscillation. More importantly, the proportions of number of pulses between π- and σ-polarized emissions were found to be integers and depended on the pump power. The pulse number ratios were consistent with the ratios of output powers of the two polarized states as shown in Fig. 4(a).
Fig. 5 Oscilloscope traces of the polarization-resolved output intensity at different pump powers: (a) Pin = 6.0 W, (b) Pin = 7.0 W, (c) Pin = 7.5 W, (d) Pin = 8.5 W, (e) Pin = 9.0 W, (f) Pin = 10.0 W.
Fig. 6 Oscilloscope traces of the polarization-resolved output intensity for optimizing operation at different pump powers: (a) Pin = 7.5 W, (b) Pin = 8.0 W, (c) Pin = 8.5 W, (d) Pin = 9.0 W, (e) Pin = 10.0 W.
This result indicated that each polarized output had the same output pulse energy but operated in opposite phase. The antiphase phenomenon of two orthogonal states was caused by the gain competition and the saturable absorber nonlinear absorption. This phenomenon has been demonstrated in Q-switched multilongitudinal-mode microchip lasers [22-24], but it has never been observed in the orthogonally polarized Q-switched Nd:YLF lasers. Finally, the expanded shapes of a single pulse for the π- and σ-polarization are shown in Fig. 7. On the whole, the pulse durations remained approximately constant at 15 ns and 11 ns for π- and σ-polarization, respectively. The pulse profile for π-polarization was found to be more asymmetric than σ-polarization. It may be caused by the cavity alignment. It is worth to mention that we don’t observed Q-switched mode-locked pulses at any point of pump power.
4. Conclusions
In summary, we have demonstrated an orthogonally polarized dual-wavelength passively Q-switched Nd:YLF laser at wavelength of 1053 nm and 1047 nm with Cr4+:YAG as the saturable absorber. The maximum overall average output power of 1.55 W is obtained at a pump power of 10.4 W under optimizing PQS operation. We have experimentally observed that the polarization-resolved pulse trains exhibited complex polarization switching dynamics which was attributed to the gain competition between two orthogonal states. It is further found that each polarized state has same pulse energy and the maximum pulse energy is calculated to be 248 μJ. The pulse durations for π- and σ-polarization are measured to be 15 ns and 11 ns and the corresponding peak power are up to 15.0 kW and 20.3 kW, respectively. We believe that our present observation is scientific interesting and can be applied for further application.
Funding
Ministry of Science and Technology, Taiwan (Contract No. MOST 105-2112-M-019-002).
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