JUPITER: Powerful impulse DPSS laser

Optical scheme

The optical layout of the laser's presented on Fig.1. Laser's made according the MOPA scheme: – Master Oscillator (MO) – 2-cascade 2-passes Power Amplifier (PA).

3 × 86 mm3 rod (АЭ) of Generator (MO) is being pumped from 3 sides with 15 laser diode bars 1500W total radiation power.

Pockels cell (E-O switch) is made on DKDP crystal base with thermostating of the crystal. The E-O gate is normally closed with high voltage 3-4.5 kV and in the moment of reaching max inversion the voltage drops for 10 ns, opening the gate. This results in short (20-25 ns) laser pulse of ~ 15 MW pulse power.

The Generator cavity is flat. But at the rate 100 Hz, it's being transformed into flat-spherical due to thermo-lensing effect causing the appearance in the rod the lense with focal length 1.4 m (at 90A pump current). The Generator laser module is located near the output mirror for increasing the output beam diameter. Intra-cavity diaphragm is absent, for due to good rod pump excitation map (it can be seen on the Photo 2, where luminescence from the top end of the rod presented ) near TEM00 mode (M2~1.5) is naturally generating. One pass weak signal generator rod amplification (at 90A) is more than 10.

Every of two Ø5 × 94 mm rods of Amplifier are being pumped from 5 sides with 25 Laser Diode bars with total power 2500W each.

The pump rod distribution for the laser modules of Amplifier is presented on Fig 3. Thermo lens - reprate frequency dependence at 90A is presented on Fig. 4.

On Fig. 5 one can see the scheme for measuring dependence of weak signal amplification - pump current in the cascade of Amplifier, and on the Fi. 6 - obtained results.

For Generator (MO) and Amplifier (PA) decoupling, as well as for protection of MO optical elements from depolarized part of PA radiation we used Faraday Cell optical insulator 9 based on permanent magnets and 6 mm aperture TGG crystal. 45 deg. quartz plate in front of it fits the azimuths of polarization of MO and FC.

1/4 wavelength phase 0-order quartz plate 13 rotates polarization plane of incident beam to orthogonal and as a result the laser radiation goes out through polarizer 4.

Fig. 1. Laser optical scheme: 1,2 - R=100% и R=30% cavity mirrors; 3 - electro-optical switch (gate); 4 - multilayer dielectrics polarizers ; 5 - R100% turning mirrors; 6 - Generator (MO) Laser module; 7 - lens with f=37 cm; 8 - plate, rotating polarization on 45 deg.; 9 - Faraday Cell on permanent magnets; 11, 12 - Amplifier (PA) Laser modules; 13 - Phase 1/4 wavelength )-order plate; 14 - R100% mirror.

Inverse population distribution along cross section of generator rod АЭ

On Fig.2 one can see the photo of luminescence intensity distribution from the top end of the MO rod and proper distribution of the intensity along horizontal axis going through the center.

Fig.2. Luminescence intensity distribution (proportional lnG, G- amplification) in arb.units from charge-coupled device pixel number in the generator rod and distribution photo.

It can be seen, that the rod excitation has rather symmetrical look with max pump intensity in the center (diameter around 2 mm).

The same distributions for PA rods are presented in the Fig.3 .

Fig.3. Luminescence intensity distribution (proportional lnG, G- amplification) in arb.units from charge-coupled device pixel number in the PA rod and distribution photo.

Thermo lens measuring results at F = 40…100 Hz

The measurements were conducted with collimated He-Ne laser beam. The distance between output end of the rod and the focusing point was measured.

Fig.4. Thermo focal length Ft (meter) for one of PA laser module via reprate frequency f (Hz)

One pass amplification

Standard method was used for measurement of cross section integral amplification - measuring weak CW signal one pass amplification. All measurements were done at 22 deg. C at pump current range 30-90A.

Results of weak signal one pass amplification of one of cascades of PA are presented in Fig.5

Fig. 5. Dependence of weak signal one pass amplification of one of cascades of PA via pump current I(A).

Measurements of energy and temporal characteristics

Output laser pulse energy 0.35J with duration 25ns at 100Hz repetition rate were obtained . MO generated pulse of 30- 40 mJ energy. Near and far radiation fields are presented in the Fig. 7.

Note that, at high reprate (100 Hz) losses via depolarization of the beam in PA caused by thermo induced birefringence became noticeable. These losses may reach 30%, and near field on the laser outputе has the look «of Maltese cross« (Fig.6).

Fig.6. Near field radiation distribution, disturbed by losses on output polarizer due to induced birefringence, which appears in active elements of PA.

The losses were practically eliminated by placing between the PA laser modules the quartz plate rotating the polarization plane on 90 deg.

Fig 7. Near field (left) and far field (right) of the laser radiation in the focus of 100 cm lens.