Lasers and OPA for optical signal amplification

The Axioma project is focused on the realization of new particle detectors characterized by low Energy threshold. The detection process is mainly based on the Infrared Quantum Counter concept (IRQC) applied to rare-earth (RE) doped crystals, as proposed by the Nobel prize Bloembergen in 1959 [1] .

Optical Amplification and Stimulated Emitted Scintillators

The aim of the research is to develop new types of low energy threshold detectors (KeV to meV) based on laser-driven transitions in active medium.

The basic idea is to use optical amplification inside a medium where inversion population is kept countinuosly on by means of a laser. The specific medium has the dual role of scintillator and active medium.

This is the starting point for examining the possibility of designing a special scintillator based on stimulated emission.

Nd:YVO4 crystal has been chosen as an active medium for both the high stimulated emission cross-section (25×10-19cm2 @ 1064 nm) and for the high sensitivity to ionizing radiation (≈104 ph/Mev @ X-ray).

Nd: YVO4 crystal absorbs photons at 808 nm populating the upper state 4F9/2 + 2H9/2, then releases into a metastable state 4F3/2 (lifetime ≈ 90 μs). From the metastable state, the crystal emits a photon at 1064 nm for spontaneous or stimulated emission. Finally the system relaxes shortly from 4I11/2 to 4I9/2 state.

This is the classic four-level laser.

Now, if population inversion is kept just below the threshold by 808 nm pump laser, even a small increase in excited states population (4F3/2) by ionizing radiation can result in optical amplification at 1064 nm.

Optical Amplification and stimulated emission set up

The experimental setup consists of a laser cavity with Nd:YVO4 crystals as active medium and a KTP crystal to generate the second harmonic (SHG at 532 nm) to facilitate signal detection.

The pump laser is a semiconductor laser coupled to a single mode and polarization manteneid fiber, maximum power of 250 mW at 808 nm.

The laser beam is collimated and focused through the high reflection mirror (HR) on the first Nd:YVO4 crystal, which is responsible for producing the laser signal at 1064 nm.

The laser threshold at 1064 nm is about 30 mW, but depends strongly on the configuration adopted.

The second Nd:YVO4 crystal can emit stimulated radiation at 1064 nm if there are excited states (i.e. if there is ionizing radiation populating the upper states) while it is transparent at 532 nm.

The output mirror (OC) reflects the 1064 nm within the cavity while transmits the 532 nm which will be the amplified signal to be detected.