Working principle X

• Continuous pumping of the laser crystal results in increasing gain and luminescence.
• If the round trip gain in the microchip laser cavity increases above one then a laser pulse starts and the absorption of the saturable absorber decreases due to its saturation. This saturation increases the gain per round trip further.
• The resonator round trip time T_R of the light in the laser cavity is very short. With a typical crystal thickness L = 200 µm and a refractive index n_e = 2.17 the round trip time T_R can be calculated to T_R = 2*n_e*L/c ~ 2.9 ps. Therefore the pulse intensity increases very fast. The slope of the increasing pulse train is determined by the modulation depth ΔR of the saturable absorber.
• With increasing pulse amplitude the stimulated emission increases. Therefore the decrease of the gain accelerates and overcomes the pump effect. If the round trip gain in case of the saturated absorber decreases below one then the laser stops.
• Because of continuous pumping after some time the gain is recovered and the laser starts again. This time between consecutive pulses decreases with increasing pump power. Therefore the pulse repatition rate is nearly proportional to the pump power.
• The onset of a pulse depends on the spontaneous emission. Therefore the repetition rate is not fixed, but fluctuates somewhat.
• If the laser cavity length L is short and the pump spot diameter small the laser works in single mode. See Applied Physics, Vol. B97, p.317, 2009 (pdf)
• The laser pulses are linear polarized parallel to the c-axis of the Nd:YVO₄ laser crystal

Pulse energy E_P X

According to the well established formalism (Journal of the Optical Society of America B, Vol. 16, Issue 3, pp. 376-388, 1999) of passive Q-switched microchip laser the pulse energy E_p can be estimated as follows:

With typical values F_sat,L = 37.3 mJ/cm², A = 1.3x10^-5cm² ( 40 µm spot diameter), ΔR = 0.1, T = 0.1, and loss A_ns = 0.05 the pulse energy can be estimated to E_p ~ 32 nJ.

Pulse repetition rate f_rep X

The average output power P_av is proportional to the pump power. Therefore the repetition rate shows a linear behavior by changing the pump power according to:

By changing the pump power P_P repetition rates between 100 kHz and 2 MHz can be realized with a nearly constant pulse energy E_P. Because the pulses start with spontaneous emission, the repetition rate shows a time jitter of about 1 %, increasing with decreasing pump power and rate.

Pulse duration t_P X

The pulse duration t_P in passive Q-switched lasers can be estimated by:

With a short laser crystal length L = 0.2 mm, refractive index n ~ 2.17, and a SAM modulation depth of ΔR = 0.1 the pulse duration is ~ 100 ps.