New UltravioleT laser System – NUTS

The NUTS project (New UltravioleT laser System) aims to develop a high power diode-pumped solid state laser in the deep UV (236,5 nm). Deep UV lasers have numerous applications in detection and sensing and in material processing. A laser at 236,5 nm can be used in detection of NO molecules (component of explosive like TNT) by fluorescence or by Raman spectroscopy.
Below 250 nm, gas laser are widely used despite their well known drawbacks (size, toxicity for excimer lasers, low efficiency, price). Standard 1 µm diode-pumped solid state lasers have difficulties to adress this wavelenth range because they require at least three non linear conversion stages.
The NUTS project proposes the development of a diode-pumped solide state laser emitting at 236,5 nm with only two non linear conversion stages. The fundamental laser is a single crystal fiber emitting at 946 nm. The targeted power is 1 W in the UV, 50 times higher than the previous state of the art.

To take this challenge up, the Laboratoire Charles Fabry (LCF) de l'Institut d'Optique and its industrial partners, Fibercryst and Cristal Laser propose to work on two keys :
- The first one concerns the laser at 946 nm with the single crystal fiber (SCF) technology. SCFs are long and thin rods (typ. 1mm in diameter and 50 mm long). Their designs allows excellent thermal management and pumping confinement. This is particularly important for 946 nm emission as the lower level of the laser transition is close to the fundamental level. First demonstrations of SCF emitting at 946 nm give results higher than the state of the art. However, to adress the challenge of the NUTS project, SCF have to go further in terms of power scaling and beam quality. The NUTS project proposes to pump the SCF at 885 nm instead of 808 nm in order to reduce the quantum defect and consequently the heating of the SCF induced by the pump by a factor higher than two. We plan to develop a Q-switched SCF laser emitting an average power of 20 W at 946 nm at a repetition rate between 20 kHz and 100 kHz with an excellent beam quality (including spatial profile and polarisation). This quality is very important for efficient non linear conversions.

- The second key is related to the non linear conversion stages. Two cascaded second harmonic generations are considered : the first one producing 473 nm emission, and the second one 236,5 nm photons. Previous works on 236,5 nm lasers reported low conversion efficiency (less than 2 %) from 946 nm to 236,5 nm. The NUTS project aims to reach an efficiency of 5%. This will be possible thanks to the quality and to the peak power of the 946 nm SCF laser. The NUTS project proposes innovative ways to use non linear crystals that can benefit to the conversion efficiency.

Both civilian and defence applications are concerned by this works :
- Remote detection of explosive or dangerous chemical agents by UV spectroscopy are clearly in the scope of the ASTRID call for proposals
- The blue and UV performance targeted in the NUTS project can adress medical applications, sub marine sensing, or material processing.
- For Fibercryst, the NUTS project is the opportunity to push the SCF technology at its limits and to offer an power scaled version of SCF modules for its customer (improvement of 2-3 in terms of output power at 1064 nm for example).
- For Cristal Laser, the NUTS project innovations may give new opportunities of markets of its crystals (LBO, RTP, KTP).