• microchip setup
• working principle
• pulse energy E_p
• repetition rate f_rep
• pulse duration t_P

The passively Q-switched monolithic microchip laser shown in the figure left consits of a Nd doped YVO₄ laser crystal and a saturable absorber mirror (SAM) , which are bonded together. The laser cavity with the length L in the region of 100 µm .. 300 µm is determined mainly by the laser crystal thickness. Typical Nd doping concentratios are 2 % or 3%. The linear polarized 808 nm pump light from a cw diode laser is parallel oriented to the c-axis af the laser crystal. A typical pump spot diameter in the laser crystal is in the region of 50 µm - 100 µm. The pulsed single mode output laser light with 1064 nm wavelength is reflected from a dichroic mirror.

• pulse energy E_p
• repetition rate f_rep
• pulse duration t_P

The SAM serves as passively Q-switch. The low intensity non-saturated SAM reflectance is in the region of 0.75 - 0.9 for the laser wavelength of 1064 nm. During the continuous pumping of the laser crystal the upper states will be filled until the gain is large enough for spontaneous emission. Then the SAM is saturated and a powerful short pulse leave the laser. During continuous pumping this development repeats with a repetition frequency f_rep inceasing nearly linear with the pump power density. Because a part of the optical energy is dissipated in the absorber layer of the SAM, arrangements have to be done to remove this heat from the microchip. 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)

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 = 0.05 the pulse energy can be estimated to E_p ~ 32 nJ.

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.

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