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The stable compact operation in the deep ultraviolet laser at 266 nm is reported from a diode-end-pumped acoustic-optic Q-switched Nd:YVO4/KTP/BBO laser. Fourth harmonic is generated by employing multiple reflections of 532 nm light for taking full advantage of the green beam. An ultraviolet laser at 266 nm with average power of 388.5 mW is obtained with a repetition-rate of 20 kHz, a pulse width of about 59.80 ns, and the corresponding optical to optical conversion efficiency of 11.9% from green to UV. High conversion efficiency and a circular spot of 266 nm laser beam have been achieved in an unfocused beam arrangement.
©2010 Optical Society of America
1. Introduction
Deep-Ultraviolet (DUV) laser could directly break apart the bones between atoms in materials. In contrast with local heating of long wavelength laser, materials would not undergo expansion and contraction in this “cold” process of dividing. Therefore, materials with high absorptivity of ultraviolet laser can be processed by short pulse-width with great precision and very little thermal effect. In addition, DUV lasers also have other important advantages such as compactness, narrow spectral width, and reliability. Nowadays DUV light sources based on nonlinear optical frequency conversion of solid-state lasers are in high demand for various applications in science and industry [1,2].
Optical frequency conversion in nonlinear crystals has become an effective assessment and versatile technique for nonlinear harmonic generation and engineering of new optical devices [3,4]. Efficient second harmonic generation (SHG) requires high power pump source and long interaction length. In single-pass generation, the optimal efficiency requires tight focusing condition to raise high power density, which limits the nonlinear crystal length and input power. Up to now, some of the solid-state DUV sources have been achieved by a single-pass harmonic generation using either a giant pulse system or a tightly focused beam arrangement [5,6]. In these sources, beta barium borate (BBO) crystal has been widely employed for frequency doubling of the visible lasers because of its physical and thermal advantages. However, the serious thermal distortion caused by tightly focused beam arrangement in high average power situation has limited its acceptance in actual applications. Moreover, the UV laser spot is flat with a rice kernel shape due to BBO’s different acceptance angles of the two polarized directions. One of the approaches to overcome these limitations is to use external resonant enhancement ring cavity in harmonic generation [7,8], which needs a piezoelectric transducer (PZT) actuator to actively control the cavity length and keep the cavity resonant with the laser frequency. Nevertheless, a ring cavity is resonantly excited by a single frequency laser beam at an accurate cavity length which is complicated in technique. In the designed multi-reflected cavity, a larger beam size is used to achieve similar conversion efficiency for the same average input and output power, as compared to the single-pass arrangement. In addition, a better beam spot of DUV laser can be obtained, which seems unattainable under a single-pass arrangement. As far as we know, high conversion efficiency together with a circular spot of 266 nm laser beam is achieved for the first time from a laser of small size at low cost.
In this paper, we describe a generation of 388.5 mW of stable 266-nm radiation with a repetition rate of 20 kHz employing a multi-reflected cavity to make full use of green laser. The efficiency of conversion from a green beam to a DUV beam is up to 11.9%. In our experiment, a low concentration continuous-grown composite crystal YVO4/Nd:YVO4 is taken as the laser crystal. The gray tracking resistance KTP (GTR-KTP) is used as the nonlinear crystal to frequency doubling from IR to 532 nm for long term use. The fourth harmonic UV generation is achieved by using a BBO crystal. A theoretical analysis is studied to prove the enhancement effect of multiple reflections. A circular spot of 266 nm laser beam is observed without using any technology to shape the laser beam. In addition, this kind of DUV lasers would be applied to clean the adherent particles from semiconductors and optical devices or detect the constituents of atmosphere in near future.
2. Design of experimental components
The theoretical study is presented on the optimization of fourth harmonic generation by multiple reflections within a cavity at 532 nm. A schematic diagram of the DUV laser system is depicted in Fig. 1 . Considering a single pulse green laser model, the intensity of incident pulse at 532 nm decreases while it travels many times though BBO crystal; meanwhile, a new pulse at 266 nm will be generated and increased gradually.
As shown in Fig. 1, a multi-reflected cavity for 532 nm is made up of mirror M3 and M1, by the latter of which an incident pulsed 532 nm beam entering the BBO crystal is reflected. Thus, the pulse intensity of green laser I1 going through the BBO crystal and being reflected once would be written as
I0 is the pulse intensity of incident beam at 532 nm. α is defined as the loss parameter which describes all diffraction and absorption losses as well as other losses except that of fundamental wave converting to the second harmonic. β is the conversion coefficient for SHG inside the BBO crystal.Similarly, pulse intensity of green laser In after multiple reflections (n is the times of reflections) inside the cavity at 532 nm is given by a recurrence relation as
Referring to the computational results introduced in Refs. [9,10], we can get the conversion factor for SHG of the SI units as followswhere L is the length of BBO crystal; deff is the effective nonlinear coefficient; c is the velocity of light; n1 and n2 are the refractive indexes of BBO at 532 nm and 266 nm, respectively; λ1 is 532 nm; ε0 is the vacuum dielectric constant.As a result, the pulse intensity of 266 nm IDUV undergoing multiple reflections is given by
To analyze the benefit of this cavity using multiple reflection process, the conversion efficiency was evaluated through a numerical analysis on the simulation of transformation from green light to DUV. The parameters used in the calculation are as follows: I0 = 3 × 106 W/cm2, L = 0.8 cm, deff = 1.75 pm/V. Owing to the impact of the walk-off angle and acceptance angle of BBO crystal, we assumed that the conversion factor β = 2 × 10−9. Obviously, the loss caused by reflection is critical to the value of α within the cavity. Based on an overall consideration of all losses, α = 0.02 would be reasonable.The theoretical pulse intensity of 532 nm and 266 nm as a function of the times of reflections is shown in Fig. 2 , from which we can see that IDUV tends to be a constant with the decrease of IGL (pulse intensity of 532 nm). The maximum limit of IDUV is 3.79 × 105 W/cm2, while the single-pass harmonic generation of IDUV is 0.18 × 105 W/cm2. We can get the enhancement factor of the conversion efficiency as follows
According to the result of numerical analysis, we are interested in the design of a multi-reflected cavity, giving a harmonic enhancement of about 21 times. Moreover, loss parameter α could be limited at smaller value to obtain higher conversion efficiency by optimizing the coating parameters. Obviously, the power of the fourth harmonic beam generated in the multi-reflection case is several times larger than that generated in the single-pass case.3. Experimental setup
Figure 1 shows the schematic illustration of the end-pumped Q-switched DUV laser. The pump source is a fiber-couple laser diode array at 808 nm wavelength with a core diameter of 200 μm and a numerical aperture of 0.22. The optical coupling system is composed of two plano-convex lenses that are antireflection coated at 808 nm. The pump beam is imaged into the laser crystal at a ratio of 1:2. The continuous-grown composite crystal YVO4/Nd:YVO4 (grown by CASTECH.INC) with a total dimension of 3 × 3 × 12 mm3 is taken as the laser crystal, among which Nd:YVO4 has a low Nd3+ concentration doping of 0.3at. % with a dimension of 3 × 3 × 10 mm3. One face of the YVO4/Nd:YVO4 coated with high transmission at 808 nm (HT, T>95%) and total reflectivity at 1064 nm (R>99.9%) is used as the former mirror of the resonator; and the other face is coated with high transmission at 1064 nm(HT,T>99%). The Acoustic-optic Q-switch (AOS, from Gooch & Housego Co.) with high transmission at 1064 nm is fixed in an adjustable mount. The GTR-KTP crystal cut for type-II phase matching (provided by Fujian ChuangXin Science and Technology Develops Co., Ltd) is 3.5 × 4.5 × 8 mm3 and both surfaces are antireflection -coated (AR,T>99.8%) at 1064 and 532 nm. The axes of KTP crystal orient at 45°to the linearly polarized face of the YVO4/Nd:YVO4. The composite YVO4/Nd:YVO4 crystal and the KTP crystal are wrapped with indium foil and mounted in a copper heat-sink, the temperatures of which are controlled at 20.0°C and 21.3°C by means of temperature controller with the accuracy of ± 0.1°C, respectively. The type-I BBO crystal for fourth harmonic generation (FHG) is 5 × 5 × 8 mm3 and both surfaces of it are dual antireflection-coated for 532 and 266 nm. For the input 532nm laser linearly polarized at 45°, the BBO crystal must also be oriented at 45°to satisfy the condition of polarization matching. The BBO crystal is also controlled at 20.0°C.
As indicated in Fig. 1, the 1064 nm resonator contains a YVO4/Nd:YVO4, an acoustic-optic Q-switch, reflecting mirror M3, a KTP crystal and coupling mirror M2 with a cavity length of 225 mm. The cavity for 532 nm is made up of mirrors M3 and M1 with a cavity length of 162 mm. Mirror M2 with its coating having high reflectivity at 1064 nm and 266 nm (HR, R > 99.9%) as well as high transmission at 532 nm (T = 98%) not only serves as the output mirror for 532 nm but also the high reflecting mirror at 266 nm. The generated DUV beam is coupled out from mirror M1 with 99.9% reflectivity at 532 nm and 94.2% transmission at 266 nm. The optical filter is HR-coated at 532 nm and AR-coated at 266 nm to obtain pure DUV output.
4. Discussion of results
Under optimized alignment, the highest DUV average power of 388.5 mW was obtained after the filter at the repetition rates of 20 kHz under the pump power of 19.4 W at 808 nm. Figure 3 plotted the measured DUV power as a function of the incident power from 0 to 20.4 W at 808 nm. The pump threshold power of DUV laser was fitted to be 5.4 W. In the same cavity configuration, we moved the BBO crystal, mirror M1 and the filter away from the experimental setup, as shown in Fig. 1. Then the output power of the pulse green laser was measured, and that versus pump power was also given in Fig. 3. The threshold power of green laser was about 5.1 W. At the highest output of 266 nm, the efficiency of conversion from a green beam to a DUV beam is 11.9%. It was noticed that the DUV output power begins to decline when the incident pump power exceeded 20.0 W. Optical power attenuation in UV region might be attributed to high two-photon absorption (TPA) of BBO, especially at multi-kHz repetition rates [11].
In this Q-switched DUV laser, the filter for 532 nm light was replaced by a CaF2 prism to precisely measure the pulse width of 266 nm light. The temporal pulse profile of 266 nm light was received by a PIN photodiode with a rising time of 0.5 ns and displayed by a Model TDS3052B (500 MHz) dual-line oscilloscope. At the average output power of 388.5 mW and the pulse repetition rate of 20 kHz, the pulse width of 59.8 ns was obtained and shown in Fig. 4 . The single pulse energy and peak power were up to 19.4 μJ and 324.8 W, respectively.
The spectrum of DUV laser was measured at the highest output power of 266 nm laser by a spectrometer (AvaSpec-3648), providing a spectral resolution of 0.08 nm. The measured spectrum of DUV light was displayed in Fig. 5 , in which the center wavelength of the second harmonic wave locates at 266 nm with a top-spike intensity distribution and the spectral width (full width at half maximum) of the second harmonic measured to be about 0.2 nm.
For a beam with Gaussian spatial distribution, the beam size at each crystal was determined using ABCD matrix method. The beam sizes located at the rear of laser medium, KTP crystal and BBO crystal were about 450μm, 230μm and 210μm respectively. The beam size at the BBO crystal is several times larger than that of tight focus arrangement. The beam spot of the 266 nm laser was photographed using a CCD digital camera and displayed in Fig. 6 . It is well known that the DUV laser spot coming out from the BBO crystal is usually flat with a rice kernel shape in the single-pass performance due to the BBO’s different acceptance angles of two polarized directions. Nevertheless, a bright circular beam spot was observed using a multi-reflected cavity in our experiment. This result indicates that the divergence angle of 532 nm laser beam is very small, so the influence of different acceptance angles on the shape of beam spot may be disregarded.
The power stability of the DUV light was measured by a Model LPM-100 power meter and the fluctuation was about 3% at the maximum output power during 30 min of operation. This fluctuation may attribute to the variation of temperature in the laser crystal and KTP crystal. Finally, mirror M1 was removed out of configuration and the filter for 532 nm light was replaced by a CaF2 prism to measure the DUV power of a single pass. Under the same condition, a single-pass power of 24.5 mW was measured. In accordance with the experimental result, we observed a harmonic enhancement of about 16 times, which showed that the actual diffraction and absorption losses α might be larger than 0.02. It was noted that the interference countervail and strengthening of multi-longitudinal mode DUV light had been omitted in the theoretical study. However, the power stability hadn’t been influenced by the interference effects, which may result from the statistical average result of these effects. The experimental results indicated that the appropriate simplification can satisfy the precision and reduce the computational complexity.
5. Conclusion
In summary, we used a multi-reflected cavity to take full advantage of the green laser, producing an efficient four-harmonic laser at 266 nm end-pumped by a laser diode. The numerical analysis of the enhancement factor of the cavity was carried out. Some of the performances of the Nd:YVO4/KTP/BBO laser and its green laser were investigated. The DUV average power of the laser at pulse repetition rates of 20 kHz is 388.5 mW with the conversion efficiency of 11.9% from a green beam to a DUV beam and the pulse width of 59.8 ns. A desirable beam spot was present without using any measures to shape the laser beam. These experimental results suggest this system approach may be used to apply DUV lasers in industry and science considering the commercial availability of the crystals, convenient operation, cost budget and other practical issues.
Acknowledgements
This research is supported by the National High Technology Research and Development Program of China under Grant No. 2006AA030107, the Guangdong-Academy Project on Industry, Academia and Research under Grant No. 2009B090600040. We thank Fujian Engineering Research Center for Laser Technology of Integration and Application (2009H2009) for the partial support.
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