High-average-power fourth harmonic generation (4thHG) of an Nd:YAG laser has been achieved by using a KBe2BO3F2-prism-coupled device (KBBF-PCD) . The highest output power of 7.86 W at 266 nm was obtained with a conversion efficiency of 10%. To our knowledge, this is the highest power ever obtained by a KBBF-PCD. The stability of the 266 nm output power at 3.26 W was measured over a period of 60 minutes, and the standard deviation jitter of the average power was 1.4%. Moreover, the temperature bandwidth for KBBF was also measured at 266nm for the first time,which shows that KBBF has significant advantages in high power 4thHG compared to other major nonlinear optical crystals and is potential for UV applications.
© 2014 Optical Society of America
All-solid-state high-power ultraviolet (UV) lasers are widely used in different fields such as precise material processing, laser marking, disc mastering, optical data storage, and spectroscopy . Compared to other UV lasers, their narrower spectral bandwidth, better optical quality, lower maintenance cost, smaller size, higher efficiency, longer lifetime, and higher stability made them playing more and more important role in industrial and scientific applications. Frequency conversion of diode-pumped high-power solid-state lasers in nonlinear optical (NLO) crystals is an attractive method of producing UV radiation. Many advances have been achieved in the development of UV laser sources, especially the fourth-harmonic-generation (4thHG) of the Nd3+-based lasers at 266nm by using NLO crystals. Thus the NLO crystals are the key factor to get high power UV lasers.
Borate crystals are often used in high-power UV generation due to their excellent NLO properties. A few excellent NLO crystals as we know, such as β-BaB2O4 (BBO), CsLiB6O10 (CLBO), K2Al2B2O7 (KABO), RbBe2BO3F2 (RBBF) and KBe2BO3F2 (KBBF) have been used for the 4thHG of Nd-based lasers (at 266 nm). The BBO has a large effective NLO coefficient, which is beneficial for high power 266 nm generation. Many results have been reported in this regard, but the output power never exceeded 3 W [2,3] until 2008, when Südmeyer et al. produced 12.2 W power in a cavity-enhanced system . Then in 2009, an output of 14.8 W was reported, which is the highest result reported for BBO so far . However, because of its relatively long cut-off wavelength (around 190 nm), it suffers from two-photon absorption during the 4thHG, which seriously decreases the conversion efficiency, degrades the beam quality, deteriorates the stability [6,7], and limits its application in high-power 4thHG finally. For CLBO crystal, there are so many results reported in increasing its output power at 266 nm [8–11]; the highest output reported in 2003 was about 40 W . However, CLBO is easily deliquescent because of its highly hygroscopic nature, which limits its applications in commercial high-power UV lasers. The 266 nm generation using KABO has also been studied in recent years , but the 266nm output power is rather low and the reason is still under investigation. RBBF crystal is a deep-UV crystal , and it also shows potential for UV applications at 266 nm. In 2012, we obtained a 266 nm UV light using RBBF crystal at a maximum output power of 3W . But for commercial application, the output power at 266 nm is looking forward to be further improved. Up till now, the highest power of commercial 266 nm lasers only reached 3 W using BBO crystals (Coherent Inc.). Commercial UV lasers are still waiting for better UV crystals.
Another candidate, KBBF, is an outstanding deep-UV NLO crystal . It belongs to the same family as RBBF crystal, and has super capabilities to produce deep-UV lasers below 200 nm. In recent years, we also discovered that KBBF is also potential for high power UV coherent generation. Table 1 lists the characteristics of KBBF and some other NLO crystals for 4thHG. It can be seen that the deff of KBBF is larger than that of KABO and RBBF, while compared with BBO and RBBF, its walk-off angle is smaller and its angular bandwidth is larger. In addition, KBBF is nonhygroscopic, and according to view mentioned in reference 7, because of its short cut-off wavelength (shorter than 160 nm), it is almost do not suffer from two-photon absorption at UV generation. It is a distinct advantage in high-power 4thHG. Moreover, the temperature bandwidth of KBBF at 266 nm is measured for the first time in this paper, and it is as high as 77 °C-cm, which is much larger than any other NLO crystals. It means that KBBF is not sensitive to temperature which is of great advantage in stable 4thHG. All these features make it a competitive candidate for producing 266 nm lasers. Some results have been reported before [18,19], but the repetition rate of the laser was only 10 Hz, and the average UV power was less than 10 mW, which is too low for most applications. In this letter, we report the use of KBBF-PCD producing 266 nm by 4thHG with a maximum output power of 7.86 W at a high repetition rate of 10 kHz. Stable operation at 3.26 W was maintained in 60 min. All these results indicate that KBBF is a potential UV crystal for high power 266 nm generation.
2. High-average-power 4th harmonic generation
The experimental setup for high-average-power 4thHG is shown in Fig. 1. The laser pump was a Q-switched Nd:YAG laser (Edgewave IS161-E) at 1064 nm with a pulse width of 10 ns and a repetition rate of 10 kHz. The maximum output power was 150 W with a beam quality M2 of 2, and the beam shape was quadrate with a size of 5 × 5 mm2. An attenuation system was used to adjust the laser power, and a lens system was used to collimate and minimize the beam diameter. For second harmonic generation (SHG) we use a LBO crystal with dimension of 4 × 4 × 30 mm3, which was cut for type I non-critical phase matching (NCPM) (θ = 90°, φ = 0°), as it has the advantage of high conversion efficiency and no walk-off effect. In the experiment, the crystal was placed in an oven and the temperature was kept at 149.5°C with a precision of ± 0.1°C. Both the entrance and exit surfaces of the crystal are antireflection coated at 1064 and 532 nm, and the spot radius of the 1064 nm beam inside the LBO crystal is 0.9mm. In Fig. 1, M1 and M2 are mirrors with high transmittance at 1064 nm and high reflection at 532 nm. A lens with a focal length of 500 mm was used to focus the 532 nm laser pump to a diameter of 0.8 mm.
The phase-matching angle of KBBF for 4thHG is 36.3°. Due to its distinct layered structure in the c (Z) direction, it is difficult to grow a KBBF crystal thick enough to be cut along the phase-matching direction . If a KBBF crystal along Z axis is directly used, the external incident angle of the 532nm laser beam would be 62.3°, so that the internal angle in KBBF would reach the phase-matching angle. In such a case, the Fresnel reflectance of 532nm beam power at the incident surface of KBBF would be 19.2% according to Fresnel’s reflection formula. To reduce the Fresnel reflection loss, a prism-coupling technique  was adopted. Figure 2 shows this sandwich structure in which the interfaces between the fused silica and KBBF are optically contacted. The apex angle of the fused silica prism was 36.9°, corresponding to the phase matching angle from 532 nm to 266 nm under the condition of normal incidence. The thickness of KBBF crystal was 2.92 mm. More details of the principle and structure of the KBBF-PCD can be found in reference 14. The KBBF-PCD was carefully rotated with a precision rotation platform to reach the phase-matching angle. The generated fourth harmonic and residual 532 nm beams are automatically separated by the rear prism in the KBBF-PCD. A power meter (3A-P-SH-V1-ROHS, OPHIR) was used to measure the 266 nm output power.
The 266 nm output power as a function of the input 532 nm power is shown in Fig. 3. It can be seen that the maximum output power reached 7.86 W when the 532 nm power was increased to 59.5 W, corresponding to a conversion efficiency of 13.4%. The conversion efficiency curve from 532 to 266 nm is shown in Fig. 4. The highest peak-power density applied in the experiment reached 124 MW/cm2. No damage was found in either the KBBF or the fused-silica prisms.
Stability, which is an important factor for applications was also performed by using another KBBF-PCD with a 2.19 mm-thick KBBF, still with the same experiment conditions. However, we changed the rear fused silica prism of the device to a CaF2 prism to avoid damage, because CaF2 has lower absorption than fused silica in the UV region. Figure 5 shows a comparison of their absorption coefficients, measured by a McPherson Model VUVaS 2000 spectrometer. First of all, we measured the stability of 532 nm, and its fluctuation is as little as 0.9% during 60 minutes, which is shown in Fig. 6. And Fig. 7 shows the stability of the 266 nm output power over a period of 1 hour. The standard deviation jitter of the average power is 1.4% at an output power of 3.25 W with a 532 nm pump power of 32.2 W. No damage to the KBBF-PCD was found during the experiment.
The experimental results for KBBF-PCD showed a conversion efficiency saturation when the input 532 nm power reached ~50 W. The reason is that such a high average power heated up the KBBF-PCD due to absorption and scattering in both KBBF and prisms. Reflection at the two interfaces between KBBF and prisms also retained idle energy within the crystal. According to the refractive indices of fused silica and KBBF, the reflection at one interface between fused silica and KBBF should be 0.03%. However, the optical contact between fused silica and KBBF is not perfect, since they are two different materials. The loss on each interface is measured to be around 1.5%. So the total power losses on the two interfaces are about 1.5 W at input power of 50 W. These may be overcome by improving the quality of optical contact and cooling the crystal system or controlling its temperature. Experiments with better optical contact and temperature control systems are still under way, and it should be possible to further improve the 266 nm power and its stability.
3. Measurement of the temperature bandwidth
Temperature bandwidth is an important parameter for nonlinear crystals, because it is closely related with the stability of harmonic generation power. To our knowledge, there is no report on temperature-dependent refractive indexes of KBBF so far. As a consequence, the temperature bandwidth of KBBF cannot be determined by theoretical calculation yet. So we measured the temperature bandwidth of KBBF for 4thHG in this paper. The light source used was a picosecond mode-locked Nd:YAG laser (PL2140 from Ekspla, Lithuania) including a SHG component which has an output at 532 nm. A KBBF crystal was placed in an oven with a precision of ± 0.1°C and its orientation angle was carefully tuned at room temperature with a precision rotation platform to reach the phase-matching angle. A prism was used to separate the 532 nm and 266 nm beam. A power meter (Physcience Opto-electronics LPE-1A) was used to measure the 266 nm power. KBBF crystal was heated up and stayed for enough time to reach thermal equilibrium. The dependence of output power at 266 nm on temperature is shown in Fig. 8. It can be seen that the 266 nm power changed slowly over a wide temperature range. We didn’t get the whole curve limited by the oven we used (the highest temperature we could get was under 200 °C). Using these experimental data, we fitted the curve and the temperature bandwidth obtained from the curve is 77 °C⋅cm. This value is quite large compared to other crystals. To further confirm the result, we also measured the temperature bandwidth of BBO using the same experimental setup. The result was 5°C⋅cm as shown in Fig. 8 together with KBBF, which is in good agreement with the reported value. This verifies the accuracy of our experimental result for KBBF. We also measured the relationship between the phase-matching angle and the temperature of KBBF, as shown in Fig. 9. It can be seen that over a temperature range of 156°C, the phase-matching angle of KBBF only changed 0.07 degree. This has confirmed the result of the temperature bandwidth, which indicates that KBBF is not sensitive to temperature, comparing to other NLO crystals like BBO, CLBO and KABO.
In conclusion, the characteristics of KBBF prism-coupled device for high-average-power 4thHG are investigated. A maximum output of 7.86 W at 266 nm has been achieved for the first time using a 2.92 mm thick KBBF crystal. The output at 266 nm can be stabilized at 3.26 W with a fluctuation of 1.4% for over 1 hour. The temperature bandwidth at 266nm is measured accurately and the relationship between the phase-matching angle and the temperature of KBBF is determined for the first time, which shows that the output power is not sensitive to the temperature variation. These results show that KBBF is a competitive nonlinear optical crystal for high-average-power 4thHG. The output power and stability may be further improved if a longer crystal is used, a better optical contact is achieved, or a thermostatic system is applied. These improvements will make KBBF more widely used in commercial laser products.
This work was supported by the National Natural Science Foundation of China (Grant No. 50972149 and 61138004), the National Basic Research Project of China (No.2010CB630701) and the National Instrumentation Program (No.2012YQ120048)
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