Miniaturized laser modules for picosecond pulses up to 2.5 W peak power in the yellow spectral range
Picosecond (ps) pulsed laser sources emitting in the yellow spectral range are demanded for several applications, especially in the field of life sciences. Innovative fluorescence imaging methods like fluorescence lifetime imaging (FLIM) or stimulated emission depletion (STED) microscopy allow for high resolution intra-cellular images, even in situ. Of special interest are pulsed laser sources with emission wavelengths at 560 nm or 590 nm that allow excitation of fluorescence without labels (e.g. label-free FLIM). Optical pulse widths of 100 ps and watt level peak powers are required, enabling also a fast scanning of the sample.
Gain-switched diode lasers are a straight-forward way for generating intense laser pulses with suitable pulse properties, offering a small footprint at the same time. However, direct emitting diode lasers for life science applications are not yet available. Thus, combining diode lasers emitting in the near infrared with single-pass second harmonic generation (SHG) is a promising approach . Over the past years, reliable high-power diode lasers at 1120 nm and 1180 nm have been developed and optimized for micro-integration at the FBH . They were used for efficient SHG, thus bisecting the wavelength.
The research during the project Yellow finally resulted in a miniaturized laser module for pulsed operation, which is shown in Fig. 1. The concept is based on a ridge-waveguide laser with distributed Bragg-reflector, which is operated in gain-switched mode for ps pulses at 1120 nm. A subsequent tapered amplifier in continuous wave operation increases the peak power of the pulses by two orders of magnitude. These laser pulses are finally coupled into a periodically poled lithium niobate crystal with ridge waveguide structure for efficient SHG. All components are mounted on a newly developed monolithic inlay for optimal heat dissipation and mechanical stability.
The finally emitted picosecond pulses at 560 nm reach peak powers of more than 2.5 W at pulse widths below 100 ps. The first module generation delivered a peak power of 0.3 W, which has meanwhile been increased by almost an order of magnitude . In Fig. 2 the dependence of the pulse peak power as well as the pulse width on the tapered amplifier current is given. While the pulse peak power strongly increases with the current, the pulse width is nearly constant. An exemplary temporal pulse shape is presented in Fig. 3 together with the emission wavelength shown in the inset. In addition to the pulse shape and spectrum at 560 nm (yellow-green lines), the pulse characteristics of a second module emitting at 590 nm are given (orange lines). At 590 nm more than 1 W peak pulse power has been achieved.
The high peak pulse power of the modules are an increase by at least a factor of 10 compared to existing ps pulse laser systems at 560 nm, offering the opportunity for fast sample scanning. Furthermore, new wavelengths like 590 nm can be addressed by the modules where currently no laser systems are available. An additional advantage is their small footprint, which allows building compact fluorescence systems.
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