1. Introduction

Continuous-wave (CW) coherent radiation in the blue and ultraviolet (UV) region is highly demanded in many scientific applications such as spectroscopy, atom cooling and trapping, and quantum optics. Because of the lack of suitable lasers, blue and UV coherent radiation has usually been generated by nonlinear frequency conversion of near-infrared (NIR) lasers. Especially, high-power UV radiation has been based on neodymium-doped solid-state lasers or ytterbium-doped fiber lasers[1, 2]. Because their wavelength is confined to 1000–1100 nm, however, they are not suitable for the generation of blue and UV radiation at 370–450 nm.

Frequency doubling of CW Ti:sapphire lasers is an ideal solution for the generation of blue and UV CW radiation because Ti:sapphire has a very broad gain profile in the NIR region of 700–1000 nm. Several groups have tried frequency doubling of CW Ti:sapphire lasers in external enhancement cavities or in intracavity doubling configurations[3, 4, 5, 6, 7, 8, 9]. In comparison with the CW UV radiation generated from neodymium-based lasers, on the other hand, the frequency-doubled power of CW Ti:sapphire lasers has been limited to below several hundreds of mW mainly because of the relatively low power of CW Ti:sapphire lasers; multi-mode 690 mW and single-frequency 266 mW were generated at 423 nm by intracavity frequency doubling with BIBO (bismuth triborate, BiB₃O₆) crystals[9], and 820 mW (630 mW coupled out) of single-frequency radiation was generated at 425 nm by external-cavity frequency doubling with a LBO (lithium triborate, LiB₃O₅) crystal[3].

In this paper we report to our knowledge on the highest powers of NIR and UV CW single-frequency radiation generated from a Ti:sapphire laser and its frequency-doubling; 5 W at 756 nm and 1.1 W at 378 nm have been generated from an injection-locked Ti:sapphire laser and its external-cavity frequency doubling, respectively. Also, we have frequency-stabilized such watt-level 378-nm single-frequency radiation for isotope-selective optical pumping of thallium atoms which are useful for the production of medical isotopes[10]and the study of parity nonconservation[11].

2. Injection-locked Ti:sapphire laser

For the development of a CW single-frequency Ti:sapphire laser, we used the technique of injection locking for the following reasons. First, injection locking does not require mode-selective optical elements such as multi-plate birefringent filters and etalons for single-frequency operation, which simplifies the configuration of ring-type Ti:sapphire lasers and thus increases the lasing efficiency with minimal intracavity loss. Second, the frequency tuning of an injection-locked laser is very easy because the wavelength of an injection-locked laser is intrinsically locked to that of a master laser; one can tune the wavelength of external-cavity diode lasers, which are most often used as master lasers, easily by moving built-in piezoelectric transducers (PZT) or controlling driving currents.

Figure 1 shows the schematic of our injection-locked Ti:sapphire laser which has been developed based on our previous work[12]. The Ti:sapphire laser, which has a ring-type configuration for the prevention of spatial hole burning, is composed of a 10-mm-long Brewster-cut Ti:sapphire crystal, two curved mirrors with a 100-mm radius of curvature, a flat mirror, and an output coupler with a 25% transmission. The Ti:sapphire crystal is 0.15-wt.% doped (FOM>150) and has a 5-mm width and a 3-mm height. Two curved mirrors have a high reflection at 756 nm and a high transmission at 532 nm, and the folding angle at the curved mirrors is set to 22° for the compensation of astigmatism in the ring cavity. The round-trip length of the ring cavity is ~700 mm, yielding ~430 MHz of longitudinal-mode spacing. The Ti:sapphire crystal, which is mounted in an aluminum block cooled by water circulation, is positioned between two curved mirrors and pumped by a frequency-doubled Nd:YVO₄ laser with a maximal power of 15 W (Verdi, Coherent). The pump laser is focused onto the Ti:sapphire crystal by a plano-convex lens of a 100-mm focal length. Because the lasing condition of the ring-type Ti:sapphire laser is very sensitive to the position of the Ti:sapphire crystal and the separation between the two curved mirrors, we mounted the crystal and the curved mirrors on linear stages to adjust their positions precisely. The flat mirror is mounted on a PZT for the control of the cavity length. Because a master laser can be perturbed or damaged by a backward laser from the ring cavity, we used an optical diode composed of a Faraday rotator and a half-wave plate for the unidirectional lasing of the ring-type Ti:sapphire laser. To roughly set the free-lasing wavelength to ~756 nm we used a 2-mm-thick single-plate birefringent filter; without the bire-fringent filter, the free-lasing wavelength was ~790 nm near the gain peak of Ti:sapphire, and the required injection power for the stable injection locking at 756 nm was increased severely.

 

Fig. 1. Schematic of the injection-locked Ti:sapphire laser.

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Our master laser is a Littrow-type external-cavity diode laser (Tiger, Sacher Laser) whose output is coupled into a single-mode fiber. The fiber-coupled output power is ~70 mW at 756 nm, and the mode-hop-free tuning range is ~30 GHz. The laser from the fiber is collimated with an aspheric lens and injected into the ring cavity. For the efficient mode matching, the separation between the aspheric lens and the fiber tip was adjusted. We use the Pound-Drever-Hall method[13] for the injection locking, and the master laser is current modulated at 38 MHz for the generation of sidebands necessary for the Pound-Drever-Hall method. A small portion of the reflected laser is monitored by a photodiode. By mixing the photodiode signal with the 38-MHz signal, we can generate an error signal for injection locking which is amplified, integrated, and used for the control of the PZT in the ring cavity. The theoretical injection-locking range calculated from the master laser’s power (70 mW), the Ti:sapphire lasers’s power (~5 W), and the cold-cavity decay rate of the ring cavity (1:1×10⁸s^-1) is 4.1 MHz.

Figure 2(a) shows the output power of the Ti:sapphire laser with respect to the pump power and the 532-nm pump absorption through the Ti:sapphire crystal. The free-running laser power without injection is 4.6 W at 15-W pumping, and the threshold pump power is 4.3 W, yielding a slope efficiency of 43%. The pump absorption is about 89% above the threshold pump power. The output power slightly increases with the injection locking; when the injection power is 70 mW, the injection-locked output power is 5 W at 15-W pumping. The slope efficiency of the injection-locked laser is 42% above 5 W of pump power. The spatial beam mode is nearly TEM₀₀ with M ² of less than 1.2 even at the highest pump power. After the injection locking, the single-frequency operation of the Ti:sapphire laser lasts over several hours even when our laser system is not vibration-isolated. The injection locking could be achieved even with 20 mW of injection power at the full-power operation, but the locking was easily stopped or perturbed by mechanical and acoustic vibrations. The output frequency of the injection-locked Ti:sapphire laser drifts slowly over several tens of MHz when the master laser’s frequency is not stabilized.

 

Fig. 2. (a) Free-running and injection-locked output power of the Ti:sapphire laser with respect to pump power and pump absorption through the Ti:sapphire crystal. (b) 378-nm power coupled out from the enhancement cavity with respect to 756-nm input power.

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3. External-cavity frequency doubling and frequency stabilization

The injection-locked Ti:sapphire laser is frequency-doubled in a commercial external enhancement cavity (WaveTrain, Spectra-Physics) which has a triangle-shape configuration composed of an input coupler, an output coupler, and a Brewster-cut folding prism. The transmission of the input coupler is ~1.3% at 756 nm, and the output coupler has high transmission at 378 nm and high reflection at 756 nm. The input and the output couplers have 50-mm and 30-mm radiuses of curvature, respectively, and the round-trip length of the cavity is ~150 mm. The folding prism is mounted on a PZT for the control of the cavity length. For frequency doubling, a Brewster-cut Type-I 10-mm-long LBO crystal with a 6-mm width and a 4-mm height is used. The cutting angle of the crystal is θ=90° with ϕ=36:3°. The beam waist at the center of the crystal calculated with ABCD matrix is ~40 µm. We mode-matched the input beam with the enhancement cavity’s fundamental mode by using a pair of plano-convex lenses. The locking of the enhancement cavity’s longitudinal mode to the input laser’s frequency is achieved by the Pound-Drever-Hall method, and an electro-optic modulator is used for the side-band generation. We installed a Faraday isolator between the Ti:sapphire laser and the enhancement cavity; without the Faraday isolator, the injection-locked Ti:sapphire laser was perturbed by backward scattered light from the enhancement cavity in spite of the unidirectional ring configuration of the enhancement cavity. The Faraday isolator induced ~15% transmission loss. Figure 2(b) shows the frequency-doubled power coupled out from the enhancement cavity with respect to the incident power. The maximal frequency-doubled power is 1.1 W at the incident power of 4.2 W, yielding a 26% conversion efficiency. Because the Brewster surface of the LBO crystal and the output coupler induce 25% loss for 378-nm radiation, the total power of the 378-nm radiation generated inside the enhancement cavity is estimated to be 1.47 W, yielding a total conversion efficiency of 35%. We believe that further optimization of the input coupler’s reflectivity can improve the conversion efficiency. In Fig. 2(b), on the other hand, one can see the slight slowdown of the increase of the 378-nm radiation above 3 W of incident power, which is believed to be caused by thermal effect in the LBO crystal.

 

Fig. 3. Schematic of the experimental setup for the generation of dispersion-like signal based on LIB: PBS, polarizing beam splitter; BS, beam splitter; PDs, photodiodes.

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For the selective optical pumping of thallium isotopes, the frequency of the 378-nm radiation should be fine tuned and locked to an atomic transition line of desired thallium isotopes which can lead to a transition to a metastable level. We use a transition of 6²P_1/2-7²S_1/2 to pump thallium atoms into a metastable level (6²P_3/2), and the laser-induced birefringence (LIB) method[14] is used for the frequency locking of the 378-nm radiation to the transition line, as shown in Fig. 3; a small portion of 378-nm radiation with linear polarization is used as a pump beam for the generation of LIB in thallium vapor, and the polarization change of a 378-nm probe beam which passes through the thallium vapor is analyzed for the generation of dispersion-like signals. In such a way, well-separated dispersion signals of thallium-203 and thallium-205 isotopes could be generated, as shown in Fig. 4(a). This signal was amplified, integrated, and used as an error signal for the stabilization of the master laser’s frequency. We measured the stability of the 378-nm frequency locking by monitoring the error signal, as shown in Fig. 4(b), and verified that the long-term frequency stability is about 3 MHz. With such frequency-stabilized 378-nm radiation, the selective optical pumping of thallium-203 and thallium-205 isotopes could be demonstrated over several hours.

 

Fig. 4. (a) Dispersion-like signals of the transition 6²P_1/2 -7²S_1/2 for thallium-203 and thallium-205 isotopes generated by the LIB method. (b) Frequency locking and stability of the 378-nm radiation.

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4. Conclusion

In conclusion, we have developed and frequency-doubled a 5-W 756-nm injection-locked Ti:sapphire laser to generate watt-level CW single-frequency radiation at 378 nm. The ring-type Ti:sapphire laser showed a slope efficiency of more than 40% and generated 5 W at 756 nm at 15-W pumping with a 70-mW injection power. An external enhancement cavity with a LBO crystal was used for the frequency doubling of the Ti:sapphire laser, and 1.1-W 378-nm radiation was coupled out from the enhancement cavity. Such high-power 378-nm radiation was frequency-locked with the LIB-based method for large-scale optical pumping of thallium isotopes, which is an important step for the production of thallium isotopes based on lasers.