1. Technical Field
The present invention relates to the field of lasers and, more specifically, passively Q-switched lasers.
2. Brief Description of Related Arts
The saturable absorber is used for generating short, high peak power laser pulses in passively Q-switched laser. Always, a cell filled with organic dye or a doped crystal is used as a saturable absorber. These optical elements have a special transmission characteristic that the transmission varies with the incident optical intensity. As the incident optical intensity increases, the saturable absorber becomes more transparent; and finally it saturates or bleaches, which leads to a higher transmission. So when placed into a laser resonator, a saturable absorber will act as a variable optical loss which means automatically modulating the Q-factor of the laser resonator. Because the saturation will occur in a very short time, which means the resonator loss suddenly reduces to a very small value, the stored energy in gain medium will build up laser radiation very quickly; also the depletion of the stored energy is very quickly, so short laser pulse with high peak power is generated.
Compared with active Q-switch, which always needs high voltage and fast electro-optic driver, passive Q-switch has many advantages, such as simple design, small size and low cost, for it is switched by laser radiation itself.
In recent years, passively Q-switched microchip lasers develop very quickly. These lasers are pumped by laser diode. Their compact size, low cost and simplicity give them potential for mass production and wider applications.
The major disadvantages of a passive Q-switch are the lack of a precision external trigger capability and a lower output compared to electro-optic or acousto-optic Q-switched lasers. The former is due to the switching by the laser radiation itself not by a precisely controllable external means. The latter is due to the residual absorption of the saturated passive Q-switch which represents a rather high insertion loss.
The most often used saturable absorber in passively Q-switched microchip laser is Cr:YAG to generate 1064 nm short laser pulses output, while Nd:YAG or Nd:YVO4 can be used as lasing medium, as described by Zayhowski in “Microchip lasers,” Optical Materials, vol. 11, pp 255-267 (1999) and U.S. Pat. No. 5,394,413. However, Cr:YAG is not suitable for Q-switching laser pulses at a wavelength longer than 1.1 μm. 1.5˜1.6 μm is known to be an eyesafe wavelength range, and in recent years, pulsed laser in this range is of great in applications of eyesafe distance measurement, lidar and optical communication.
Cobalt doped solid state material, is now often used as saturable absorber to obtained laser pulse output at wavelength ranged from 1.3 μm to 1.6 μm. Compared with uranium doped CaF2 or other cobalt doped crystal e.g. Co:LMA, which are also often used as saturable absorber in 1.5 μm, cobalt doped spinel crystal has a larger ground state absorption cross section and smaller excited state absorption cross section, which means easier to achieve short pulse width and high pulse energy level.
In many applications of passively Q-switched laser, a fast response photodiode is needed to receive the laser pulses signal and converts it to electrical signal for triggering purpose.