Periodically Poled KTP Waveguide Crystal Generates Blue Light

Breck Hitz

A collaboration of researchers at the University of St. Andrews in the UK, the University of Sofia in Bulgaria and AdvR Inc. in Bozeman, Mont., has demonstrated a frequency-doubled laser capable of producing ultrashort pulses of 424-nm light with an overall electrical-to-optical ("wall plug") efficiency of 0.5 percent. Although several members of the collaboration have previously demonstrated a femtosecond blue source capable of higher overall efficiency, the new laser achieves a higher slope efficiency and may prove to be a better source of ultrashort blue pulses for applications such as high-density optical storage, biomedical imaging and the study of protein dynamics.

Figure 1. The 848-nm pulses from the mode-locked Cr:LiSAF laser (within the dotted lines) are frequency-doubled in a single pass through the periodically poled KTP crystal.

The new laser is a mode-locked Cr:LiSAF laser whose output is frequency-doubled in a single pass through an external periodically poled KTP crystal (Figure 1). Diode lasers at 660 and 685 nm pump the Cr:LiSAF crystal, which is mode-locked with a semiconductor saturable absorber. An intracavity prism provides wavelength tunability and group-velocity dispersion compensation. The laser's 848-nm pulses emerge through a 1.5 percent output coupler.

A single aspheric lens focuses the pulses into a periodically poled KTP waveguide. The KTP waveguide, which had been fabricated for a different experiment, includes a Bragg grating section next to the waveguide section. The Bragg grating is a detriment to the present experiment, reflecting as much as 26 percent of the incident 848-nm light. Fortunately, this backreflected light does not degrade the performance of the Cr:LiSAF laser. As much as 5.6 mW of average power at 424 nm is generated in the KTP when the input from the Cr:LiSAF laser is 27 mW. The total electrical power to the diodes in this case is 1.2 W, so the overall electrical-to-optical efficiency is approximately 0.5 percent.

Although the external second-harmonic conversion efficiency (5.6 mW of blue from 27 mW of infrared) is 21 percent, when the loss due to the Bragg grating and the Fresnel loss from the uncoated crystal are taken into account, the internal conversion efficiency is 37 percent. The internal conversion efficiency increases with input power, up to a maximum of 37 percent, but it saturates at higher input powers (Figure 2). The slope efficiency, defined here as the slope of the conversion efficiency plotted against input power, is 5.5 percent per picojoule for input powers lower than the saturation value.

Figure 2. The internal conversion efficiency increases with input power to a maximum of 37 percent, after which it saturates, probably due to two-photon absorption of the blue light.

The international collaborators who produced these results believe that this is the best slope efficiency yet reported for conversion into the blue spectral region. The researchers believe that the explanation for the saturation of the slope efficiency at higher input powers is two-photon absorption of the second-harmonic wave.