• #11-2. Operation principles and method of different lasers



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    The passive Q-switching technique uses simpler resonator architecture compared to the active Q-switching technique and can be manufactured in a smaller device but cannot arbitrarily control the repetition rates. Unlike the above mentioned free running or long pulsed lasers, the Q-switching technique requires addition of following elements to the resonator structure; an optical element controlling the polarization direction and a switching element (Pockels cell.) controlling the resonator loss. Therefore, the resonator design greatly influences the characteristics of the laser output. This is an important factor that attests to the technological skills of the manufacturer.


    [Ad. ▶HYPERION(Nd:YAG) - Manufacturer: LASEROPTEK(www.laseroptek.com)

    HELIOSⅡ/LOTUSⅡ/HYPERION – Manufacturer: LASEROPTEK(www.laseroptek.com)


    As discussed earlier, YAG stands for “yttrium aluminum garnet” and is a synthetic crystalline form of garnet. The metal oxide form is sapphire and addition of chrome during crystallization results in ruby. And addition of neodymium (Nd) results in Nd:YAG. This transparent YAG crystal is used as a gain medium in a laser and this type of laser is called YAG laser.

    In a YAG laser, it is the small amount of neodymium that provides the lasing activity, not the YAG crystal. The gain medium (or the active laser medium) is a medium that can amplify the power of light. Just as a transistor (amplifier) is needed in the electronic circuit of a cordless phone to produce the radio wave, an amplifier is needed for laser oscillation.

    The YAG crystal has excellent chemical stability and does not change in direct contact with water. It also has optical consistency. In particular, its excellent heat conduction allows quick cooling and thereby continuous high power output. For this reason, a YAG laser equipped with a YAG crystal as the gain medium is mainly used in a metal cutter, welding machine, medical equipment, laser marker, and academic equipment that require a large power output.

    Most lasers produce light energy that is less than 2% of the energy input. Poor energy efficiency mainly stems from optical pumping. Optical pumping is the process of pumping light into the atoms of the gain medium (e.g. Nd) to amplify the energy level. Light is generated from an object when atoms move from a high energy state to low energy state and emits the residual energy in the form of radiation.

    In terms of the process of light generation, laser does not differ much from other illuminants. However, not many options exist for stimulating excitement in the atoms of the laser gain medium. If the gain medium is a conductor or in the rare form of gas, it can be pumped through collision with electrons, however, for a transparent gain medium, optical pumping is the only method of inducing the excited state of atoms.

    First, an electric light emitting device is needed to optically pump the gain medium. The most suitable light emitting device for a YAG laser is a discharge tube filled with xenon gas.

    The efficiency of this discharge tube is at most 30% and the residual energy is consumed in the form of heat which has longer wavelength. Only a very small amount of this 30% contributes to optical pumping and the rest is absorbed into adjacent objects to turn into heat energy. The reason for this is that the Nd atoms in the gain medium travel energy levels in a discontinuous way, similar to the time change shown in a digital clock as opposed to the continuous hand movement of an analog clock. Light with medium wavelengths are dispersed and only the light with wavelength equal to energy level difference is absorbed. Most light emitting devices, as well as a xenon discharge tube, emit light with continuous spectrum and therefore, have poor efficiency when used in optical pumping. For these reasons, the light source for laser optical pumping should be monochromatic light rather than white light and even though monochromatic light, it should be absorbed by the gain medium.


    -To be continued-


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