An example of compact picosecond diode-pumped seed laser

Pic.1 An optical scheme of a picosecond seed laser.

In this article we would like to give a principal scheme for fast saturable absorber mode-locked Nd-doped solid state seed laser for solid state Nd:YVO₄ amplifiers. We will also try to highlight features to pay attention at in order to avoid wasting time and ruining your equipment.

Fiber-coupled high power laser diode bar (λ=808nm) is used as a pumping source and an a-axis cut Nd-doped (0.5%) as a gain medium. The front face of the gain medium has a dielectric coating with high reflection coefficient for λ=1064nm and high transmission coefficient at λ=808nm. The rear face is declined by 8 degrees relatively to the front surface and has an anti-reflection coating (0.1%) to decrease feedback, avoiding mode-locking disturbances.

The pair of aspherical lenses (F=8mm) is used to couple the pumping beam into the gain medium so that the mean radius of the excitation mode is less than 250μm. It is a good practice to adjust a short linear cavity with an active element and simple 90% plane mirror just behind the output surface of the active element first and use the radiation of this short laser as guidance when adjusting the long cavity. If this assistant cavity is aligned, it is easy to make other aligning processes.

The laser resonator is designed using the -500mm and -600mm curvature radius mirrors so that the mode diameter in the gain medium is equal to 250μm and in the passive absorber (SESAM) it is equal to 85μm. So, on the one hand, the mode diameter corresponds to the gain range in the gain media and the saturation fluence for the absorber is provided on the other hand. Fine tuning of the length of the SESAM leg of the cavity is needed to find mode locking, cause it critically influences the mode diameter in the SESAM

The semiconductor saturable absorber mirror (SESAM) in the schema has 4ps relaxation time and 3% reflection coefficient modulation depth. The saturation fluence for the absorber is 25μJ/cm².

Until the saturation fluence is reached, the laser works in the giant pulse generation mode with a several hundreds of nanoseconds duration of envelope, filled inside with an ultrashort pulse train. When the radiation energy density in the resonator exceeds the gate saturation density, the laser enters the regime of passive mode locking generating similar extremely short pulses.

Pic.2 The endless train of similar extremely short pulses generated by seed laser in the regime of passive mode locking. The repetition rate was 99MHz.

The resonator design works very stable and supports a lot of longitudinal modes and may support also few transverse modes. Always check spatial modes with M2 meter, because mode-locking may be disturbed by higher order modes. The use of SESAM mirror allows to avoid the difficulties with the adjusting of resonator length or compensation of non-linearities (for fiber lasers with soliton mode-locking the time to achieve the stable regime could be in order of half an hour). The known parameter to be adjusted is the pumping power for achieving stable mode-locking when the ambient temperature changes by more than 2 degrees. If the ambient temperature is stable, laser runs without need for adjustment for months and even years.

This simple scheme provides two mode-locked 150-170mW output beams at pump power about several Watts. Exact values depend strongly on absorption efficiency, mode-matching of the pump and laser beams, parasitic loss in your cavity and your alignment of the beam waist on the SESAM.