Dichroic mirror

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Mirrors are most commonly used to direct the laser beam towards the work piece at a 90° bend (45° angle of incidence). With careful coating design, reflectivities approaching 100% are common. Because the coating is on the front surface of the mirror and reflects nearly 100% of the laser beam, the thermal properties of the substrate material are not critical. Some beam delivery systems are designed to allow a camera to view the work piece through the back side of the bending mirror. In this case, the substrate must be transmissive in the visible spectrum and typically a broad band anti-reflective (BBAR) coating is applied to the back side for improved visible transmission and camera imaging. Many laser systems incorporate a HeNe or diode laser for aligning the beam and locating the focused spot. These systems must use dichroic bending mirrors which reflect both the primary 1064nm/1070nm wavelength and the red 632nm HeNe wavelength or 670nm diode wavelength. Due to design constraints, reflectivity can only be maximized at one wavelength. Therefore, dichroic mirrors are usually designed for maximum reflectivity (nearly 100%) at the primary wavelength and best effort high reflectivity (usually 80%-90%) at the alignment laser wavelength.