10W, high repetition rate, 775 nm fiber laser with high resolution pulse shaping, and on-demand pulse to pulse switching capability, for bioinstrumentation
Louis Desbiens, Vincent Roy, Michel Jacob, Yves Taillon
Summary form only given. Advances in the research fields of biological, biophysical and biochemistry rely on the development of novel, flexible, powerful and reliable laser sources in the visible – NIR part of the spectrum [1, 2]. Optical excitation in the 750-800 nm region, with flexible pulsed formats, can be advantageous in applications such as confocal fluorescence microscopy, STED, FLIM microscopy, photoluminescence spectroscopy, laser photocoagulation and time-resolved spectroscopy. Finely tailored pulse formatting can indeed provide significant enhancement in many of those techniques [3] by optimizing the temporal distribution of the optical excitation with respect to the specific characteristics of the fluorophore of interest such as its excited-state lifetime for example. With those considerations we have developed a fiber laser source that combines a 1550 nm pulse-shaped MOPA system [4], offering coarse temporal control resolution (1 ns), with a high frequency AWG module operating at close to 8 GHz, that drives an intensity modulator downstream of the directly modulated semiconductor seed diode of the MOPA system. The result is both an improvement of the temporal control resolution (130 ps) and an increase of the possible dynamic range by combining the pulse shaping capability of the two sub-systems. The polarization-maintaining, LMA fiber-based, 20W IR laser source is frequency converted to 775 nm in a MgO:PPLN crystal which exhibits a conversion efficiency as high as 65% (see inset of Fig. 1(a)). More importantly, it can sustain a conversion efficiency of greater than 35% over most of the temporal range of operation (70 ps – 6 ns), while preserving an excellent M2 of better than 1.1 (see inset of Fig. 1(b) and (c)). The repetition rate of the source is nominally set at 10 MHz but can be adjusted as the AWG module and the pulsedshaped MOPA system share a common clock and are arranged in a master/slave configuration. Finally the design of the system allows for rapid switching of the active pulse shape without any interruption of the pulse train which could prove to be very useful for, on the fly, optimization of the pulse shape during a continuous scan. Fig. 1(d) illustrates such rapid change of the active pulse shape at up to 10 MHz. Any of the 255 different pulse shapes held in memory can be called at any given time, the switch occurs after completion of the active pattern generation.
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