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Laser mirrors

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    Definition:
    High quality mirrors used in laser resonators and other optical devices.
    The laser resonator contains a mirror, which needs to meet some requirements,
    especially the following:
     Low reflection loss (for high mirrors), or a specified transmittance over a given
    wavelength range (for output couplers)
     High surface optical quality (very flat over a large area,avoid wavefront distortion that
    degrades the beam quality)
     Can withstand high light intensity, with high damage threshold, avoid laser-induced
    damage (especially in q-switched lasers)
    In most cases, a multi-layer dielectric reflector (usually a quarter wavelength reflector) is
    used as the laser reflector. (The specific preparation details of dielectric laser mirrors are
    given in dielectric coatings.)Typically, one laser mirror is used as an output coupler with
    high transmittance to the laser radiation, while others are highly reflective (reflectivity
    >99.8%). Some mirrors may use dichromatic mirrors so that the pump light can enter the
    gain medium of the end-pumped laser. (For quasi three level lasers, because the pump
    light and laser wavelength are very similar, it is necessary to use this dichromatic mirror.)
    The laser mirror can also reflect light from outside the laser resonator. For example, a pair
    of tilted mirrors is often used, each reflecting at an Angle close to 90° to reflect the laser
    beam to other devices. The adjustment price of the inclined mirror usually has two or three
    adjusting screws, which can adjust the starting point and direction of the beam.
    Mirror substrate
    Typically, laser mirrors are prepared on a glass substrate (for example, BK7 or fused
    quartz), but the mirror coating can also be directly coated on the laser crystal, for example,
    a monolithic laser. Usually the mirror substrate is elliptical in shape with a size of 25.4mm
    or 12.7mm and a thickness of 6mm. Even for high mirrors, the properties of the substrate
    are very important, especially the surface quality and high rigidity, low thermal expansion
    coefficient, high thermal conductivity (avoiding thermal expansion in high power lasers).
    For partially transmitted reflectors, high optical uniformity (avoiding beam distortion of
    transmitted light), low absorption and scattering losses are also required.

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