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Microlaser bicolore à haute fréquence pour le STED simplifié

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Background & general objectives
Microchip based microlaser is a perfect choice for the conception of a STED microscopy system. Its sub nanosecond pulse duration is well adapted to the dynamics of the most popular fluorescence contrast agents. Due to its wavelength conversion scheme based on harmonic conversion in extra cavity nonlinear crystals, both excitation and depletion wavelength are intrinsically synchronized and aligned. In addition, microlaser based sources are compact, robust and less expensive than other sources used in commercial STED.

The speed of image acquisition has to be considered since we are concerned by imaging of living cells. For faster imaging each pair of pulses gives one pixel information, and the speed of the imagery processing is directly proportional with the frequency of the laser source pulse emission. For this project, Teem Photonics is studying a new design of microlaser that can overcome the actual limitation of its commercial laser source in term of repetition rate (current maximum 140 kHz) and reach up to 1 MHz.

Design and study
The new laser source has been designed and studied on an open bench. It can be divided in three different parts

Schematic diagram of the laser source

-Seed infrared laser
The infrared seed laser source is a solid pump Passively Q-Switched (PQS) microlaser cavity composed by Nd:YAG as amplifying medium and a semiconductor saturable absorber mirrors (SESAM) as saturable absorber. The microlaser chip can produce 1064 nm pulses of less than 200 ps duration at a repetition rate of 1 MHz .
- Fibre Amplifier.
A Fiber Amplifier stage is then added in a Master Oscillator Fiber Amplifier (MOFA) architecture in order to deliver more than 0,6 µJ at 1 MHz repetition rate of 180 ps long infrared pulses at the output.
- Conversion stage
The infrared 1064 nm is then converted to its harmonics through the pass in two different non linear crystals in critical phase matching conditions. The output is finally filtered in order to obtain both 532 nm and 355 nm simultaneous pulses. Care is taken to compensate the walk off in the second nonlinear crystal than separate the two wavelengths by using a silica slab.

The high frequency microlaser open bench showing the different stages

At the output, we obtain simultaneous pulses at 1MHz repetition rate with and energy of 50 nJ at 532 nm and 2,5 nJ at 355, nm with nearly perfect Gaussian beam (M² <1,1), compatible with the STED microscopy application.

Infrared prototype realization
We have designed and built a compact prototype of this infrared amplified laser source and its controller. The dimensions of the laser head are 5 cm x 21 cm x 22 cm. It can deliver up to 0.8 µJ at 1 MHz with nearly perfect Gaussian beam quality. A conversion stage module will be added in order to obtain the double wavelength output 532/355nm as demonstrated in the open bench.

The high frequency infrared amplified micro laser and its controller