Introduction

Due to the steadily increasing demand of the multimedia applications powerful RGB (red green blue) light sources came into the focus of industrial research. Along this road the Praseodymium laser has been reinvented again since this material has the potential to emit directly visible laser radiation on many interesting wavelength.

Whereas in the past this material has been of more scientific interest it is nowadays considered as a noteworthy candidate for RGB applications. The recent new developments of compact Pr:YLF laser have been enabled due to the presence of powerful blue emitting laser diodes. Such blue laser diodes actually have been developed for the powerful RGB data projectors.

Basic set-up with hemispherical cavity

The Pr:YLF crystal is optically pumped with the blue (448 nm) radiation of a a laser diode (BLD). The emission is collimated by a short focal lens (CO) and subsequently focused with a lens (FL) having a focal length of 50 mm into the Pr:YLF crystal. The Pr:YLF crystal is located close to the beam waist of the optical cavity formed by the plane mirror M1 and the curved mirror M2. The Pr:YLF laser crystal is mounted into a rotary mount since the excitation efficiency depends on the polarization of the pump light. The filter (FI) is used to suppress not absorbed pump radiation, typically an RG495 color glass filter.

Set-up of a spherical cavity with two curved mirrors

By using two curved mirrors M1 and M2, a spherical cavity is realized. The Pr:YLF laser crystal is located apart from the in-coupling mirror M1. The mirror have a radius of curvature of 100 or 150 mm, so that also a combination of 100 and 150 mm can be used. In this case one needs to to take care about an optical instable range.

This configuration provides more space inside the cavity to accommodate more intra-cavity elements like second harmonic generation crystals (see Fig. 3.)

Intra-cavity second harmonic generation of 640nm to 320 nm

The strong radiation at 640 nm allows the efficient frequency doubling or second harmonic generation of UV radiation to 320 nm. Such UV radiation is of great importance in bio-photonics.

A LBO (Lithium triborate) crystal is used as frequency doubler. It is 8 mm long with a quadratic cross section of 3 mm. The crystal is cut for Type I phase matching and is applied intra-cavity.

Setup with two plane mirrors using an intra-cavity lens

The cavity is extended by the lens (IL) resulting in an almost parallel beam required for the operation with the Littrow prism or of the plane mirrors. The Littrow prism (LP) is coated with a broadband coating having a high reflectivity >99.98 % in a range of 580..725 nm. 5 visible lines can be obtained 606, 639, 676, 697 and 720 nm.

laser line tuning with an intra-cavity birefringent tuner (BFT)

The birefringent tuner (BFT) is inserted into the cavity between the intra-cavity lens (IL) and the flat mirror M2. 5 visible lines can be obtained 606, 640, 676, 697 and 720 nm.

Setup to measure the fluorescence of the optically pumped Pr:YLF rod

This simple set-up uses a photodiode and the filter FI to suppress not absorbed pump power. The controller can switch the diode laser (BLD) periodically on and off. This allows the measurement of the life time of the excited states.