We report the generation of quasi-continuous-wave vacuum-ultraviolet (VUV) coherent light based on a Ti:sapphire laser with two successive frequency doubling stages. In the first stage, UV light at 399 nm with power of 1.1 W was obtained by exploiting an enhanced cavity. With a KBBF crystal as nonlinear material, quasi-continuous-wave VUV coherent light with power of about 25 mW at 199.5 nm and 4.7 mW at 193.5 nm were achieved through a single-pass SHG configuration, respectively, in the second stage.
©2009 Optical Society of America
Vacuum-ultraviolet (VUV) coherent light has been proposed for different applications, as optical data storage, metrology, biomedical application, fundamental spectroscopic research, and laser lithography. Conventional coherent VUV laser sources at this wavelength are excimer lasers (ArF and KrF). These lasers generate a high output power of more than 100 W, however their structures are huge and complex. Moreover, high manufacturing and maintenance costs are required. The low repetition rate (usually several kHz) of such excimer laser systems also restricts the applicability of many shot statistics during data acquisition in spectroscopic measurements. Another way to produce coherent radiation in this wavelength range consists in the optical frequency conversion of solid-state laser in nonlinear optical crystals, which are transparent in the VUV, such as BBO, LBO or other borates. The development of coherent radiation with high-repetition rate (quasi-continuous wave) or continuous wave (cw) in VUV has to be based on the frequency conversion or exploitation of new laser materials. At present, the later is a more challenging topic and the former is the unique way to generate VUV laser light with high-repetition rate or continuous wave. Generally, techniques of optical frequency conversion for generating short wavelength light are second harmonic generation (SHG) and sum frequency generation (SFG) process . There are several commercial nonlinear optical crystals that are capable of producing VUV coherent light, such as LBO and BBO, through SFG . But these crystals cannot be used to produce light at wavelength shorter than 200 nm through SHG because it is impossible to achieve the phase-matching angle of SHG. Compare to the setup of SHG, the optical setup of SFG required multiple laser sources; consequently, the system was slightly complex and multiple optical beams coupling also makes system at low efficiency.
Thanks the development of material science, the new excellent nonlinear material, which is named KBe2BO3F2 (KBBF) , makes it possible to generate light at wavelength shorter than 200 nm through SHG. Based on Sellmeier equations, it was predicted that VUV light at wavelength as short as 165 nm could be generated through direct SHG with KBBF as nonlinear crystal . In experiment, VUV light at wavelength of 170 nm has been obtained through a direct SHG with a KBBF crystal . Meanwhile, VUV light at wavelength of as short as 157 nm has also been produced by fifth-harmonic generation of a Ti:sapphire laser with a KBBF as nonlinear crystal . In the past decade, both the generated average-power and conversion efficiency have been significant improved . However, the quality of the KBBF crystal severely restricted the laser sources to pulse lasers especially with low repetition rate benefit from the high-peak power of the each pulse. Recently, the research began to focus in the generation of quasi continuous wave VUV light with the development of the KBBF crystal and prism coupling technique (PCT) [8–11].
Enhanced resonant cavity is another most important technique for an increase in frequency conversion efficiency, especially for continuous wave lasers  and mode-locked pulse lasers . Without additional costs in terms of laser power, the intensity power can be coherently boosted in the cavity; thus providing much average or higher peak power for more efficient energy conversion. Conversion efficiencies of more than 70% have been obtained for frequency doubling both the continuous wave laser and mode-locked pulse lasers by exploiting an enhanced cavity. Our group is now involving the generation of more than 100 mW quasi-continuous wave VUV laser light with two successive SHG stages in the enhanced resonant cavity, in which LBO and KBBF are used as the nonlinear medium respectively, from a mode-locked Ti:sapphire laser. In this paper, we report on the first step of this project, namely, the realization of frequency doubling of Ti:sapphire laser with a enhanced resonant cavity and generation of VUV coherent light by a single pass through SHG system with KBBF as nonlinear material. Best to our knowledge, quasi-continuous-wave VUV with power of about 25 mW is the highest power at present by means of frequency conversion.
2. Experimental setup
In Fig. 1 we show a schematic diagram of our experiment setup. The fundamental beam is provided by a mode-locked pulsed Ti:sapphire laser, whose center wavelength was 798 nm. The average output power was typically about 1.7 W. The output of Ti:sapphire laser is passed through a lenses, which was used to manipulate the q-parameter of the fundamental beam before the resonant cavity, and introduced into the enhanced SHG cavity. There is a general limitation when one uses a mode-locked laser. The free spectral range of the enhancement cavity must match (or be a subharmonic of) the free spectral range of the laser cavity; it is not a free parameter. In our experiment we used a laser that emitted 1.5 ps pulses at an 82 MHz repetition rate. So the cavity length was first set at approximately 3656 mm, and then finely adjusted to the point of maximum enhancement. The cavity, as the same as the enhanced cavity in Ref. 13, consists of two flat mirrors (M1 and M2) and two curve mirrors (M3 and M4). M1 was a coupling mirror, whereas M2-M4 were coating for high reflectivity at the fundamental wavelength. Several different input couplers, with transmission values between 5% and 30%, were used to couple the fundamental light into the cavity. In addition, M4 was also highly transmitting (80%) at the SH of 399 nm. The radius of curvature of M3 and M4 was 200 mm, and the circulating beam inside the ring cavity was focused between them. At the focus between M3 and M4, a 15 mm lithium triborate (LBO) crystal was chosen as the SHG martial because of its large nonlinear coefficient and low walk-off effect. A small leak through M3 is monitored by a calibrated photodiode (not shown) to measure the power Pc circulating in the cavity; this photodiode is also used to align the cavity. To keep the external doubler cavity enhancement of the laser pulses in a simple and stable system, we had to control the external cavity round-trip length to the longitudinal modes of the Ti:sapphire laser cavity. For this purpose, the mirror, M2, was mounted on a piezoelectric transducer (PZT). The initial adjustment of the round-trip length was done with a translational stage. We did this by monitoring the temporal resonance and the resultant enhancement of the circulating beam while scanning the PZT-mounted mirror. Locking was performed by a Hansch-Couillaud locking scheme, which was performed by a quarter-wave plate (QWP), a polarization beam splitter (PBS), and a pair of detectors D1, and D2. The error signals were generated with a differential amplifier and feedback to a PZT driver through servo loop.
At the second stage, we generate the VUV laser light by single-pass configuration SHG, in which an enhanced resonant cavity is not employed. The generated UV light from enhanced SHG cavity, as shown in Fig. 1, is focused into a KBBF crystal through a lens. Output power of the VUV light is measured by a power meter. KBBF crystal is an excellent VUV nonlinear crystal. However, the KBBF crystal has a plate like form along the z-axis of the crystal and its thickness is extremely limited around 2~3 mm. Hence, a special prism coupling technique  is necessary to avoid cutting the crystal along the phase-matching direction for SHG. The KBBF crystal is sandwiched between two fused-silica prisms, which have almost the same refractive indices with KBBF in UV region, through optical contact. The whole device has been named KBBF PCT device and they have been successfully used in the VUV laser generation in recent years [8–11]. In our experiment, a PCT device, which consists of a KBBF with thickness of 2.3 mm and a pair of fused-silica prisms with an angle of 55 degree, is employed. Phase matching angle of KBBF in type-I SHG depends the fundamental wavelength. To obtain the perfect phase matching angle, the incident angle of fundamental is slightly adjusted from zero degree since the PCT device is designed for SHG performance near 400 nm. In experiment, it was realized by mounting the PCT device on a rotating stage. The incident angle can be adjusted by rotating the stage. Generated VUV and fundamental UV wave can be separated though the rear prism based on the different refractive indices.
3. Experimental results
In experiment, it is impossible to get a perfect mode matching for the injected fundamental wave; we extended the standard equations to account for the mode-matching factor m (0 < m <1) . The SH output power (P 2), reflected fundamental power (Pr), and the circulating fundamental power (Pc) inside the cavity are given by,
where P 1 is the fundamental power incident on the cavity, ENL is the single-pass conversion efficiency, L is the round-trip intracavity loss and T is the transmission of the input coupler. ENL can be measured by removing the input coupler of the enhanced cavity, with a result that ENL =0.12 ± 0.02W-1. We have tested three different cavities with different input coupler (T=5%, 10%, and 30%). Using the above formulas, the intracavity loss and mode-matching factor can be calculated. It resulted in estimation of L = (1.3 ± 0.2)% and m = 0.70 ± 0.04, respectively. Figure 2 gives the output SH average power as a function of the reflectivity of the input coupler. The solid curve is the theoretical prediction using the above-mentioned parameters. The generated UV with different input coupler is also plotted in the figure. There is good agreement between the experiment and theory, resulting that the estimation of intracavity loss and mode matching factor were reasonable.
Except for the above-mentioned parameters, ENL, L, and m, the performance of the doubling cavity also depends essentially on the impedance matching and power of fundamental wave. The impedance matching can be achieved when the input coupler transmission equals the sum of all other losses, including the power-dependent conversion losses, Topt =L + ENL Pc. For our estimated parameter values Topt is calculated to be 37% at the fundamentals power of 1.7 W. Since the enhanced cavity is a low finesses cavity, the output SH power is insensitive the transmission of the input coupler near the impedance matching condition. It also be indicated in Fig. 2. The difference of output SHG power is less than 10% when the transmission of input coupler belongs between 30% and 40%. In our following experiment, an input coupler with transmission of 30% is employed.
Figure 3 gives the input-output characteristics and conversion efficiency of the external resonant doubler from 798 nm to 399 nm, also plotted in the figure are our experimental results for P 2 versus P 1 when the external cavity was locked to an incident fundamental resonance. From the measured values for ENL, and estimated values for m, and L, the theoretical prediction for the harmonic output P 2 and conversion efficiency are also shown in Fig. 3 as the fundamental power P 1. More than 1.1 W of output at 399 nm was observed with the maximum input of 1.7 W and a maximum efficiency was 65% . If the reflective loss (20%) at the output mirror was accounted for, the power generated in the crystal was estimated to be 1.37 W, corresponding to 80% conversion efficiency. It shows good agreement between the experiment and theory. Hence it is reasonable to say that the impedance matching was nearly optimized for this experiment. At present, the conversion efficiency, we believe, was limited only by the mode-matching factor. With a perfect mode matching (m→1) we would expect to achieve more than 90% conversion efficiency. The output beam pattern at 399 nm was elliptic suffering from the walk-off effect in nonlinear crystal. In the far field, the beam pattern has an aspect ratio of approximately 1:6. The compensation of walk-off and improvements of the generated beam pattern are in considering and processing.
Figure 4(a) gives output power curves of VUV laser light, which was generated by single-pass through SHG using KBBF as nonlinear crystal from 399 nm to 199.5 nm with a focused lens of focused length 200 mm; and Fig. 4(b) shows the conversion efficiency. A maximum output power of about 25 mW, corresponding to conversion efficiency of 2.2%, was obtained at the input UV fundamental power of 1.1 W. We did here the same behavior: the curve in Fig. 4(a) was well fitted by Pout =E NL2 Pin 2 with an E NL2 = 0.02W-1. Meanwhile, a linear fitting the conversion efficiency gives an E NL2 of 0.019W-1, which is agree well with the above result, in Fig. 4(b). From Fig. 4(b), it also shows that the conversion efficiency is not saturated yet, and higher conversion efficiency and output power could be obtained if more fundamental power at 399 nm is applied. A direct conversion efficiency of 1.5% from Ti:sapphire laser is obtained. The further increase of conversion efficiency and output power could be realized by establishing a resonant enhanced cavity.
We also obtained VUV output at 193.5 nm from the fourth-harmonic generation of Ti:sapphire laser. To do this, the wavelength of Ti:sapphire laser was tuned from 798 nm to 774 nm. The generated 387 nm UV laser light was as the fundamental input incident into the KBBF crystal. Once again, the phase matching angle for SHG can be realized by slightly adjusting the angle of incidence. Since the enhanced resonant cavity is designed for SHG from 798 nm to 399 nm, external losses of the resonant enhanced cavity at 774 nm became larger and only 400 mW of output power at 387 nm was obtained. The VUV light at 193.5 nm with a output power of 4.7 mW (corresponding to conversion efficiency of 1.1% ) was obtained with fundamental input power of 400 mW at 387 nm.
In summary, we have demonstrated an all-solid-state VUV coherent light source by two successive second harmonic generation from a Ti:sapphire laser system. With an enhanced cavity, about 80% SHG conversion efficiency was achieved for quasi-continuous-wave laser light. Maximum output power of 25 mW at 199.5 nm was also obtained by single-pass through SHG with a KBBF as nonlinear crystal at fundamental power of 1.1 W.
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