I. Introduction
Recently, DUV light sources have also been rapidly developed because of their usefulness in a large number of industrial applications and scientific fields. Nowadays, the EFHG performed by the Nd-doped crystal laser at 1064 nm provides a convenient and reliable way to obtain the DUV radiation at 266 nm, which intrinsically takes the advantages of higher efficiency, longer lifetime, higher stability, easier implement, and smaller system size etc. The principle of the FHG with the widely used Nd-doped crystal at the 4F3/2 → 4I11/2 transition is based on the cascaded processes of the SHG.
The first stage is converting the fundamental IR beam to the visible green light by frequency doubling of the Nd-doped crystal laser at 1064 nm, and one subsequently performs another SHG of the generated 532-nm radiation to acquire the DUV laser at 266 nm [23-28].
In this section, based on the green lasers discussed in Sec. 4.1, we perform the EFHG to verify that the ESHG is more advantageous in generating DUV laser at 266 nm than the intracavity one, where the conversion efficiencies from 532 to 266 nm for the two cases are 37.1 and 7.2 %, respectively. Moreover, the output power at 266 nm as high as 1.67 W is effectually generated with the combination of the ESHG and EFHG under an incident pump power of 26 W at 808 nm and a pulse repetition rate of 40 kHz, corresponding to the conversion efficiency from 808 to 266 nm of up to 6.4 %.
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II. Experimental setup
Among several nonlinear crystals for realizing an efficient EFHG at 266 nm, we chose the BBO crystal because of its high nonlinear coefficient and low hygroscopic property. The dimensions of the BBO crystal were 3 × 3 × 10 mm3, and it was used as the type-I phase-matched condition at room temperature along the direction at = 47.6˚, ϕ = 0˚. Both surfaces of the BBO crystal were AR coated at 1064, 532, and 266 nm. A convex lens with a focal length of 19 mm was employed to focus the green light into the BBO crystal for realizing an efficient EFHG operation. Both sides of the convex lens were AR coated at 1064 and 532 nm. Although the same optical components for the two configurations were exploited, the EFHG processes were individually optimized for the ESHG or ISHG. The geometrical distance of L5 and L6 indicated in Fig. 4.3.1 were 25 and 22 mm for the ESHG, while for the ISHG they were 16 and 30 mm.
Fig. 4.3.1. Arrangement of the experimental setup for the EFHG.
BBO Convex lens
Diode-pumped AQS Nd:YVO4 green laser at 532 nm
L5 L6
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III. Conversion efficiencies in the EFHG process with ESHG and ISHG Figure 4.3.2 graphically summarizes the output characteristics at 266 nm by frequency doubling of the green lasers at 532 nm obtained with the ESHG and ISHG.
Note that the incident powers at 532 nm for the ESHG are 30-40 % lower than those for the ISHG, as illustrated in Fig. 4.1.2(a). However, it is apparent that the conversion efficiencies in the EFHG process obtained with the ESHG are noticeably higher than the results obtained with the ISHG, as exhibited in Fig. 4.3.2(a). For the ESHG, the highest output power at 266 nm of 1.67 W is achieved under the maximum incident power of 4.5 W at 532 nm. The corresponding conversion efficiencies from 808 to 266 nm and from 532 to 266 nm are up to 6.4 and 37.1 %, respectively. Note that the conversion efficiencies from 808 to 266 nm and from 532 to 266 nm obtained with the previous studies on the DUV generation at 266 nm were not more than 3 and 20 % [25-28]. The high optical conversion efficiency achieved in this work is believed to come from the optimization of the AO Q-switched laser without the parasitic lasing effect as well as the use of the convex lens to focus the green light into the BBO crystal. On the other hand, the highest output power at 266 nm for the ISHG is only 0.46 W under the maximum incident power of 5.76 W at 532 nm. The corresponding optical conversion efficiencies from 808 to 266 nm and from 532 to 266 nm are 1.8 and 7.2 %, respectively.
The more efficient DUV attainment obtained with the ESHG is definitely due to the fact that the considerably higher peak power greatly enhances the conversion efficiency in the EFHG process as compared with the ISHG.
The pulse energies at 266 nm versus the pulse repetition rate for both cases are illustrated in Fig. 4.3.2(b). The pulse energy at 266 nm decreases from 51 to 6.5 μJ for the ESHG and from 15 to 1 μJ for the ISHG by increasing the pulse repetition rate from 30 to 100 kHz. Figure 4.3.2(c) presents the dependences of the peak power at 266 nm on the pulse repetition rate for both cases. When the pulse repetition rate increases from 30 to 100 kHz, the peak powers for the ESHG and ISHG vary from 7.3 to 0.4 kW and from 1.2 to 0.1 kW, respectively. Owing to the combined effect of the large walk-off property of the BBO crystal and the tight-focusing scheme of the extracavity harmonic generation, the beam quality factors for the two cases are both found to be relatively poor; that is, Mx2 ~ 2, and My2 ~ 1.5. On the whole, we have experimentally manifested that using the ESHG to perform EFHG can provide a superior laser performance at 266 nm in output power, pulse energy, and peak power in comparison with the ISHG. We
believe that the comparison presented here can give important insights into the fields of the laser technology and the harmonic generation.
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Fig. 4.3.2. Dependences of the (a) output power, (b) pulse energy, and (c) peak power at 266 nm on the pulse repetition rate.
Pulse repetition rate (kHz)
20 40 60 80 100 120
Output power at 266 nm (W)
0.0
Pulse energy at 266 nm (J)
0
Peak power at 266 nm (kW)
0
4.4 Conclusion
Efficient extracavity harmonic generations performed with optimized Q-switched Nd-doped crystal IR lasers are successfully achieved. First of all, we properly design an efficient and reliable single-end-pumped AQS Nd:YVO4 laser to make a systematic comparison of the output performance at 532 nm between the extracavity and intracavity configurations in the SHG process. Experimental investigations reveal that the output power and pulse energy obtained with the ISHG are higher than the results obtained with the ESHG under the same incident pump power and pulse repetition rate.
Nevertheless, the relatively wide pulse duration accompanied with a long tail due to the high finesse of the laser cavity is experimentally found to lead the peak power obtained with the ISHG to be significantly lower than that obtained with the ESHG. Under an incident pump power of 26 W at 808 nm, the maximum output powers of 4.5 and 5.76 W and the largest pulse energies of 133 and 177 μJ are accomplished for ESHG and ISHG, whereas the highest peak powers obtained with ESHG and ISHG are found to be 30.4 and 11.6 kW, respectively.
In Sec. 4.2, utilizing the developed PQS Nd:YVO4 laser to perform the ESHG and ETHG, the maximum output powers at 532 and 355 nm are found to be up to 2.2 and 1.62 W with a pulse width as short as 5 ns and a pulse repetition rate of 56 kHz. The largest pulse energy and the highest peak power at 355 nm are found to be 29 μJ and 5.8 kW, respectively. The optical conversion efficiencies from 1064 to 355 nm and 808 to 355 nm are up to 26 and 10 %, respectively. To our knowledge, this is the highest conversion efficiency for the 355-nm UV laser generated by the PQS Nd:YVO4/Cr4+:YAG laser.
We then employ the optimized AQS Nd:YVO4 laser to achieve highly efficient extracavity harmonic generations. At an incident pump power of 44 W, the output powers at 532 and 355 nm under a pulse repetition rate of 40 kHz reach 8.38 and 6.65 W, respectively. In addition, at a pulse repetition rate of 20 kHz, the largest pulse energy and the highest peak power at 355 nm are found to be 200 μJ and 22 kW, respectively. The optical conversion efficiencies from 1064 to 355 nm and from 808 to 355 nm are found to be up to 38.2 and 15.1 %, respectively. This is the highest conversion efficiency for the 355-nm UV radiation based on the AO Q-switched laser
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with a flat-flat cavity to date.
The PQS Nd:YLF laser developed in Sec. 2.7 is also successfully applied in the processes of the extracavity harmonic generations to produce green and UV radiations, where the largest pulse energies of 490 μJ at 527 nm and 360 μJ at 351 nm are efficiently generated with the shortest pulse width of 4 ns.
Finally, following the works in Sec. 4.1, we further verify that the higher peak power obtained with the ESHG is more helpful for the 266-nm generation via the EFHG process as compared with the ISHG, where the conversion efficiencies from 532 to 266 nm are 37.1 and 7.2 % for the ESHG and ISHG, respectively. In addition, under an incident pump power of 26 W at 808 nm and a pulse repetition rate of 40 kHz, the maximum output power of our Q-switched Nd:YVO4 DUV laser at 266 nm based on the combination of the ESHG and EFHG arrangements reaches 1.67 W, corresponding to the conversion efficiency from 808 to 266 nm up to 6.4 %. To the best of our knowledge, this is the highest conversion efficiency at 266 nm ever reported among the continuously pumped Q-switched Nd-doped vanadate DUV oscillators with the same incident pump power at 808 nm.
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Chapter 5
Summary and Future Works
5.1 Summary
High-power Q-switched Nd:YVO4 lasers at 1064 nm and high-energy Q-switched Nd:YLF lasers at 1053 nm have been optimized to exhibit excellent output performance.
We have considered the second threshold criterion and the thermal-lensing effect to design a high-peak-power PQS Nd:YVO4 laser with the Cr4+:YAG crystal as a saturable absorber. At an incident pump power of 16.3 W, the output power was found to reach 6.2 W with a pulse width of 7 ns and a pulse repetition rate of 56 kHz. The corresponding pulse energy and peak power were as high as 111 μJ and 16 kW, respectively.
In Sec. 2.3, we have explored the parasitic lasing effect in an AQS laser with a flat-flat resonator and a 0.1 at.% Nd:YVO4 crystal. Experimental results revealed that the critical cavity length without parasitic lasing was proportional to the pump power.
The parasitic lasing effect was also found to lead to a long tail in the Q-switched pulse, corresponding to a reduction in the peak power. We manifestly disclosed that the combined effects of the parasitic lasing and the thermal lens made Nd:YVO4 crystals with dopant concentration greater than 0.2 at.% to be problematical in designing the high-power Q-switched laser with a flat-flat cavity. After optimizing the AQS laser, the maximum output power of 19.4 W was obtained at 100 kHz, while the shortest pulse width of 8 ns, the largest pulse energy of 650 μJ, and the highest peak power of 81.5 kW were accomplished at an incident pump power of 44 W.
In Sec. 2.5, we have successfully demonstrated a reliable TEM00-mode linearly polarized laser at 1053 nm with the natural birefringence of a wedged Nd:YLF crystal in a compact concave-plano cavity. Using the Cr4+:YAG saturable absorber to perform PQS operation, the maximum output power can be up to 2.3 W under an incident pump power of 12 W. Under this output condition, the pulse repetition rate and the pulse width were found to be 8 kHz and 9 ns, respectively. The corresponding pulse energy and the peak power were up to 288 μJ and 32 kW, respectively. We believe that the relatively compact configuration presented here is potentially useful for the generation of high-peak-power pulses in Q-switched Nd:YLF lasers at 1053 nm.
With c-cut Nd:YLF crystal, we have found that decreasing the ROC of the concave mirror can usefully extend the power scale-up for a laser in a concave-plano cavity to be
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influenced by a large negative thermal lens. With this finding, we have developed a practical tactic to scale up the output power of a compact high-pulse-energy PQS Nd:YLF laser at 1053 nm with the Cr4+:YAG crystal as a saturable absorber. At an incident pump power of 12.6 W, the optimum PQS laser at 1053 nm emitted the maximum output power of 2.61 W with a pulse width of 6 ns and a pulse repetition rate of 4.6 kHz. The corresponding pulse energy and peak power were up to 570 μJ and 95 kW, respectively. We further experimentally confirmed that the negative focal length of the thermal lens is considerably enhanced by the ETU effect in the PQS Nd:YLF laser.
Moreover, we have theoretically realized and experimentally designed a nearly hemispherical cavity to scale up the pulse energy of a nanosecond pulsed pumped PQS Nd:YLF/Cr4+:YAG laser with the pulse repetition rate tunable from 100 Hz to 1 kHz. At a pulse repetition rate of 100 Hz, the largest pulse energy and the shortest pulse duration were achieved to be 1.38 mJ and 5 ns with this compact pulsed laser.
The AO Q-switch has been utilized to achieve AQS operation in the c-cut Nd:YLF crystal with the pulse repetition rate ranging from 5 to 40 kHz. Under an incident pump power of 12.7 W, the maximum output power of 4.5 W was fulfilled at 5 kHz, whereas the shortest pulse duration of 25 ns, the largest pulse energy of 800 μJ, and highest peak power of 32 kW were accomplished at 40 kHz. In addition, we have exhaustively explored the influences of the thermal effect and anisotropic property of the AO Q-switch on the polarization characteristics of the c-cut Nd:YLF laser in the CW and Q-switched operation for the first time.
In Chap. 3, a novel concept of a separable monolithic IOPO cavity was originally proposed. With the developed approach, we have remarkably improved the stability and efficiency of the IOPO driven by a diode-pumped Q-switched Nd:YVO4/Cr4+:YAG laser. The mirrors were directly deposited on the facets of the KTP crystal to form an independent monolithic IOPO cavity with high stability and high optical conversion efficiency. The performances of IOPO with Rs = 80 and 50 % have been investigated.
The output powers at 1552 nm were up to 3.3 W at the maximum incident pump power of 16.8 W for both cases with the instability of 0.2 % for Rs = 80 % and 1 % for Rs = 50
%, respectively. The diode-to-signal conversion efficiency was up to 20 %, which is the highest one for IOPOs to our knowledge. At the maximum pump power, pulse energies were 40 μJ at a pulse repetition rate of 80 kHz for Rs = 80 % and 45 μJ at a pulse
repetition rate of 68 kHz for Rs = 50 %, respectively. The temporal domain showed that several satellite pulses were observed behind a major pulse for Rs = 80 %. Reducing the Rs to 50 %, satellite pulses could be suppressed effectively without energy loss. The pulse train amplitude fluctuation in standard deviation was slightly larger with the lower Rs. However, the peak power was remarkably enhanced by employing the KTP crystal with lower reflectivity of the output coupler at 1552 nm, which is advantageous to generate high-peak-power eye-safe light source.
With the same design concept, we have demonstrated an efficient high-pulse-energy eye-safe radiation in a Nd:YLF/KTP IOPO with the help of thermally induced polarization switching. We properly measured the temporal behaviors of the depleted fundamental pulses and manifestly found that the thermally induced birefringence can lead the mutually orthogonal polarization states of the fundamental pulses to be effectively switched for accomplishing an efficient IOPO operation without any extra polarization control. With this finding, the pulse energy as high as 306 μJ with the optical conversion efficiency up to 12.3 % was achieved in our compact
With the same design concept, we have demonstrated an efficient high-pulse-energy eye-safe radiation in a Nd:YLF/KTP IOPO with the help of thermally induced polarization switching. We properly measured the temporal behaviors of the depleted fundamental pulses and manifestly found that the thermally induced birefringence can lead the mutually orthogonal polarization states of the fundamental pulses to be effectively switched for accomplishing an efficient IOPO operation without any extra polarization control. With this finding, the pulse energy as high as 306 μJ with the optical conversion efficiency up to 12.3 % was achieved in our compact