• #13-2. Operation principles and method of different lasers Ⅲ



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    Transverse electric field

    As shown in <figure 14-2>, the direction of transverse electric field applied to the crystal is perpendicular to that of propagation. Here, the electric field is applied to the side of the crystal and as shown in the right drawing of <figure 14-2>, a small aperture in the form of waveguide has the benefit of operation at a lower voltage.


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    One important property of a Pockels cell is half-wave voltage. This type of voltage is necessary in changing the phase of incident light by π (180°). For example, in an amplitude modulator, half-wave voltage should be applied on a crystal in order to transform light from minimum transmission to maximum transmission.

    When transverse electric field is applied, the half-wave voltage of Pockles cell varies depending on the physical quality of crystal and distance between electrodes. For example, for a large effective aperture, the strength of applying voltage should increase in proportion to the distance between electrodes.

    As Pockels cells generally require half-wave voltage of hundreds to thousands of volts, a high-voltage amplifier is needed to maintain large modulation depths. However, with LiNbO₃, a crystal with strong nonlinearity or an integrated optical modulator with narrow electrode spacing, low voltage can be used. But such devices produce low output of light.


    <Figure 14-2> Pockels cell with transverse electric field. The left is a bulk modulator and the right is a waveguide modulator.


    Wave plate used in Q-switching

    A wave plate is a clear crystal material with finely adjusted birefringence which is used to modulate the polarization state of light. The polarization state is determined by the direction of electric field oscillation and can be divided into four categories as below.


    Horizontal linear polarization

    Vertical linear polarization

    Left circular polarization

    Right circular polarization


    A retarder plate has a slow axis and fast axis perpendicular to the direction of the light beam and these two axes are also perpendicular to each other (as in a 3-dimensional coordinate system of XYZ). The role of a wave plate is to modulate the phase velocity of the light propagated on the wave plate. The polarized light propagated along the fast axis has slightly higher velocity.

    Therefore, the retardance value is determined by the wave plate design and is very sensitive to the bandwidth and the angle at which light is propagated onto the wave plate.

    The most widely used wave plates are quarter-wave plates (λ/2 plates). Quarter-wave plates signify that the phase delay difference between linear polarization of fast axis and slow axis is π/2 and half-wave plates is π.

    For example, when linearly polarized light passes through a quarter-wave plate, it becomes circularly polarized and contrarily, circularly polarized light becomes linearly polarized.

    As the phase difference changes by π with a half-wave plate, perpendicular linearly polarized light becomes parallel linearly polarize, left-circularly polarized light becomes right-circularly polarized.

    Generally, a crystalline quartz (SiO₂) is used as a wave plate due to its high transmission rate at a wider bandwidth and high optical quality.

    Other crystalline materials such as calcite (CaCO₃), magnesium fluoride (MgF₂), sapphire (Al₂O₃), mica (a silicate material), or birefringent polymer, etc. is used depending on the wavelength of the light used.

    Wave plates such as half-wave plates are very thin as phase retardation should be π. This creates manufacturing and handling challenges,

    Therefore, wave plates are attached to a thick glass panel or sandwiched between two glass sheets to allow safer handling. But this also has to consider mechanical stability of the glass as it is sensitive to temperature change.


    -To be continued-


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