• #14-2. Operation principles and method of different lasers (4)



    ▶ Previous Artlcle : #14-1. Operation principles and method of different lasers (4)


    To operate laser, the power supply unit is essential and the most important part of power supply is the design of optimal PFN (pulse forming network). PFN consists of RLC circuit, SCR or spark gap, and a switching element such as Thyratron. Here, the RLC circuit determines the time of flash lamp firing and discharge and controls the electricity (discharge) applied to the gain medium or light (flash lamp) energy level. Therefore, if the design of these elements are not optimal, the efficiency of laser oscillation is lowered. Especially, as excimer laser or nitrogen laser uses very short discharge time (ns), the inductance and capacitance values should be balanced in the design. The performance of a laser is determined by the laser head and power supply, both of which are of equal importance. For example, the laser performance is compromised if the laser head is optimally designed but the power supply is flawed. Therefore, expertise in laser as well as electricity are needed to manufacture a high-performance laser.


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


    Long pulse laser

    Long pulse lasers have a longer pulse duration compared to free running lasers. Laser is oscillated over a longer period of time (>10⁻³ seconds), which is determined by PFN in power supply. Laser is oscillated when the gain is always bigger than or equal to loss in the resonator. For this, sufficient energy should be applied to the gain medium in PFN over a long period of time. Solid-state lasers (Nd:YAG/Alexandrite laser, etc.) are generated with a more powerful capacitor (by approximately 150 times) and correspondingly larger power supply. Therefore, long pulse laser and free running laser can be said to have similar laser oscillation principle except for the power supply.

    There are two goals of increasing the power of capacitor. This allows longer discharge time and delivery of sufficient energy to the gain medium. Discharge time is proportional to inductance(L) multiplied by capacitance(C) (√LxC) and the energy level is in proportion to capacitance (E=CV²/2, V:applied voltage).

    In general, long pulse solid-state laser has low peak power of the output pulse, therefore an ordinary optical fiber can be used to deliver laser beam. This produces a very uniform beam of laser as laser beam is combined to the optical fiber. However, if the core diameter and divergence angle of laser are not considered in coupling of the laser beam with the optical fiber, the laser output can be very inconsistent. In a long pulse laser, this is caused by poor pointing stability (degree of change in laser beam direction due to thermal lens effect) of the laser beam. The core diameter of the optical fiber should be selected with caution as the pointing stability and the size of focused laser beam change depending on the output.


    -To be continued-

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