Compact violet lasers by second-harmonic generation in KTP waveguides

Periodically segmented waveguides were fabricated in flux-grown KTP for quasi-phasematched second-harmonic generation (SHG) of a light beam with a wavelength of 425 nm. Diode-pumped violet laser sources are proposed on the basis of these waveguides. We shall show that a pulsed operation of the pump diode laser at a 940 MHz repetition rate enables the construction of sources with a very compact geometry, which are insensitive to temperature fluctuations. These sources may still be considered as quasi-continuous wave (cw) for applications in high-density optical recording.

The most compact type of violet laser source has a size of 1 × 1 × 2 cm³. It contains only the diode pump laser, the KTP waveguide and a miniature lens to couple the pump beam to the waveguide. Time-averaged violet output powers up to 85 μW have been generated for many hours at room temperature without requiring an active temperature control. This output power may be sufficient for reading an optical disc.

By optical feedback of a portion of the transmitted pump beam via an external grating it is possible to generate higher violet powers. In this way, the pump laser is forced to operate in a single spectral mode, the wavelength of which can be tuned to coincide with the phase-matching wavelength of the waveguide. This grating-controlled laser system is shown to generate a 425 nm beam with powers up to 0.5 mW. The total length of the device is about 7 cm.