**4. UV light generation by SHG**

UV light sources are interesting in many fields such as biophotonics/chemical photonics, material processing, and optical data storage. Although diode lasers based on AlGaN have been demonstrated around 340 nm in pulse mode recently (Yoshida et al., 2008a, 2008b), frequency doubling of a red laser beam through a nonlinear crystal is still an efficient method to generate CW light in this wavelength range (Mizuuchi & Yamamoto, 1996; Mizuuchi et al., 1997, 2003a, 2003b; Knappe et al., 1998). UV light around 340 nm has been achieved by single pass SHG in bulk periodically poled LiTaO3 (Mizuuchi & Yamamoto, 1996; Mizuuchi et al., 1997) and MgO:LiNbO3 (Mizuuchi et al., 2003a) crystals and also achieved in a periodically poled MgO:LiNbO3 ridge waveguide (Mizuuchi et al., 2003b); but so far these periodically poled devices are not commercially available, and no UV light shorter than 340 nm has been demonstrated with these first-order periodically poled devices.

Here UV light around 337.5 nm will be generated using the external-cavity tapered diode laser system developed above as a pump source. The generated CW UV light source will be used as the excitation source for fluorescence diagnostics. Compared with other UV laser sources around 337 nm, such as a CW krypton-ion laser and a pulsed nitrogen laser, the UV laser source based on a tunable tapered diode laser system is far more simple, compact, and easy to operate.

A BIBO nonlinear crystal is used for frequency doubling of the 675.2 nm red light to the 337.6 nm UV light due to its relatively high effective nonlinear coefficient. The 10 mm long BIBO crystal with an aperture of 4 mm × 4 mm is cut with *θ* = 137.7º and *φ* = 90º for type-I phase matching (*eeo*) and antireflection coated on both end surfaces for 675/337.5 nm. The spectral bandwidth of both laser system B and C is narrow enough for frequency doubling through the BIBO crystal (the acceptable spectral bandwidth of this crystal is around 0.2 nm). Laser system C is chosen as the pump source for the frequency doubling experiment due to its better spatial beam quality.

Furthermore, we compare the results from the three laser systems based on different tapered gain devices. This is important for us to choose tapered gain devices for different

Parameter A B C Max. power (W) 1.38 1.25 1.05

power (nm) 668.35 675.34 675.67 Tunable range (nm) 659-675 663-684 666-685

(nm) <0.07 <0.05 <0.07

Table 2. Summary of the main parameters for diode laser system A, B and C.

*M*2 value 2.00±0.01 (1.27 W) 2.07±0.02 (1.0 W) 1.13±0.02 (0.93 W)

UV light sources are interesting in many fields such as biophotonics/chemical photonics, material processing, and optical data storage. Although diode lasers based on AlGaN have been demonstrated around 340 nm in pulse mode recently (Yoshida et al., 2008a, 2008b), frequency doubling of a red laser beam through a nonlinear crystal is still an efficient method to generate CW light in this wavelength range (Mizuuchi & Yamamoto, 1996; Mizuuchi et al., 1997, 2003a, 2003b; Knappe et al., 1998). UV light around 340 nm has been achieved by single pass SHG in bulk periodically poled LiTaO3 (Mizuuchi & Yamamoto, 1996; Mizuuchi et al., 1997) and MgO:LiNbO3 (Mizuuchi et al., 2003a) crystals and also achieved in a periodically poled MgO:LiNbO3 ridge waveguide (Mizuuchi et al., 2003b); but so far these periodically poled devices are not commercially available, and no UV light shorter than 340 nm has been demonstrated with these first-order periodically poled

Here UV light around 337.5 nm will be generated using the external-cavity tapered diode laser system developed above as a pump source. The generated CW UV light source will be used as the excitation source for fluorescence diagnostics. Compared with other UV laser sources around 337 nm, such as a CW krypton-ion laser and a pulsed nitrogen laser, the UV laser source based on a tunable tapered diode laser system is far more simple, compact, and

A BIBO nonlinear crystal is used for frequency doubling of the 675.2 nm red light to the 337.6 nm UV light due to its relatively high effective nonlinear coefficient. The 10 mm long BIBO crystal with an aperture of 4 mm × 4 mm is cut with *θ* = 137.7º and *φ* = 90º for type-I phase matching (*eeo*) and antireflection coated on both end surfaces for 675/337.5 nm. The spectral bandwidth of both laser system B and C is narrow enough for frequency doubling through the BIBO crystal (the acceptable spectral bandwidth of this crystal is around 0.2 nm). Laser system C is chosen as the pump source for the frequency doubling experiment

Laser system

applications.

Wavelength with max

Spectral bandwidth

devices.

easy to operate.

due to its better spatial beam quality.

**4. UV light generation by SHG** 

A 30 dB optical isolator is inserted between the aspherical lens and the cylindrical lens in the output beam to avoid feedback from the optical components and the nonlinear crystal, as shown in Fig. 6. A biconvex lens of 75 mm focal length is used to focus the red fundamental beam into the BIBO crystal. The available output power of the fundamental beam in front of the crystal is 650 mW. The size of the focus *w*s × *w*f is around 70 µm × 35 µm, where *w*s and *w*f are the beam waists (diameters at 1/e2) in the slow and fast axes, respectively. The elliptical beam is used to reduce the effects of walk-off in the BIBO crystal. The walk-off angle in our crystal is 72.9 mrad, corresponding to a heavy walk-off parameter *B* of 15.1. The elliptical beam was proved to be optimum in the experiments, in good agreement with the theory of frequency doubling using elliptical beams (Boyd & Kleinman, 1968; Steinbach et al., 1996). The slight change in astigmatism with output power will cause the focusing conditions to vary slightly at different power levels. In the experiments, the astigmatism was corrected at maximum pump power. Two dichroic beam splitters separate the fundamental beam from the second harmonic output beam.

The wavelength of the fundamental beam is tuned to 675.16 nm, and the temperature of the crystal is 19.8 ºC. Figure 12 shows the measured second harmonic power as a function of fundamental power. The curve represents a quadratic fitting. A maximum of 109 µW UV light is obtained with a fundamental pump power of 650 mW. The conversion efficiency *η* is 0.026%W-1, compared to a conversion efficiency of 0.019%W-1 for a single-pass frequency doubling through a 15 mm long LiIO3 bulk crystal (Knappe et al., 1998), and the theoretically calculated value is 0.040%W-1 (Steinbach et al., 1996).

Fig. 12. Second harmonic power as a function of fundamental power. The squares are measured data, and the curve is a quadratic fit.

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