A cylindrical lens is a lens that focuses light into a line, as a spherical lens would. The curved face or faces of a cylindrical lens are sections of a cylinder, and they focus the image passing through them into a line parallel to the intersection of the surface of the lens and a plane tangent to it. The cylindrical lens compresses the image in the direction perpendicular to this line and leaves it unaltered in the direction parallel to it (in the tangent plane). Cylindrical lenses focus or expand light in one axis only. They can be used to focus light into a thin line in optical metrology, laser scanning, spectroscopic, laser diode, acousto-optic, and optical processor applications. They also can be used to expand the output of a laser diode into a symmetrical beam. In telecom, cylindrical lenses are widely used in WSS, 40G/100G modules, and pump laser modules.