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N-BK7 Plano-Convex Lenses (High Power, V-Coated: 633 nm)
These Plano-Convex Lenses are fabricated from N-BK7 glass and feature laser line V-coats at 633 nm for use with popular high-power HeNe lasers with pulsed outputs up to 10 J/cm2. N-BK7 is a common optical glass that can be used for high-quality optical components. It is typically chosen whenever the additional benefits of UV fused silica (i.e., good transmission further into the UV and a lower coefficient of thermal expansion) are not necessary.
These plano-convex lenses are popular for many applications. They have a positive focal length and near-best-form shape for infinite and finite conjugate applications. Plano-convex lenses focus a collimated beam to the back focus and collimate light from a point source.
To minimize the introduction of spherical aberration, a collimated light source should be incident on the curved surface of the lens when being focused and a point light source should be incident on the planar surface when being collimated. The focal length of each lens can be calculated using a simplified thick lens equation. f= R/(n-1), where n is the index of refraction and R is the radius of curvature of the lens surface. These lenses are fabricated from N-BK7, which has an Abbe Number of 64.17; this value is an indicator of the dispersion.
With a reflectance of less than 0.25% at the coating wavelength, these lenses provide exceptionally efficient transmittance and are ideal for use with HeNe lasers, as well as applications where light is transmitted through complex optical systems. Durable and capable of withstanding up to 10 J/cm2 (10 ns, 10 Hz), these V-coated spherical singlets are also particularly well-suited for high-power applications.
Below is the transmission curve for N-BK7, a RoHS-compliant form of BK7. Total Transmission is shown for a 10 mm thick, uncoated sample and includes surface reflections.
633 nm V-Coat Reflectance (AOI: 0 - 20°)
The plot on the right is an enlarged view of the shaded region:
Click to Enlarge
Click Here for Raw Data
In the thick lens equation, use the index of refraction for N-BK7 at the wavelength of interest to approximate the wavelength-dependent focal length of any of the plano-convex lenses.
The focal length of a thick spherical lens can be calculated using the thick lens equation below. In this expression, nl is the index of refraction of the lens, R1 and R2 are the radii of curvature for surfaces 1 and 2, respectively, and d is the center thickness of the lens.
When using the thick lens equation to calculate the focal length of a plano-convex lens, R1 = ∞ and R2 = -R. Note that the minus sign in front of R is due to the sign convention used when deriving the thick lens equations. Therefore, via substitution, the thick lens equation becomes
The focal length of the lens calculated using the simplified thick lens equation directly above is the distance between the second (back) principal plane (H") and the position at which a collimated beam incident on the curved surface of the plano-convex is focused. The principle plane positions of a thick lens can be calculated with the following equations:
However, as with the thick lens equation, H' simplifies to zero and H" simplifies to
when used to calculate the principle plane locations of plano-convex lenses. fb is the back focal length of the lens, which is often referred to as the working distance of the lens.
Mounting High-Curvature Optics
Thorlabs' retaining rings are used to secure unmounted optics within lens tubes or optic mounts. These rings are secured in position using a compatible spanner wrench. For flat or low-curvature optics, standard retaining rings manufactured from anodized aluminum are available from Ø5 mm to Ø4". For high-curvature optics, extra-thick retaining rings are available in Ø1/2", Ø1", and Ø2" sizes.
Extra-thick retaining rings offer several features that aid in mounting high-curvature optics such as aspheric lenses, short-focal-length plano-convex lenses, and condenser lenses. As shown in the animation to the right, the guide flange of the spanner wrench will collide with the surface of high-curvature lenses when using a standard retaining ring, potentially scratching the optic. This contact also creates a gap between the spanner wrench and retaining ring, preventing the ring from tightening correctly. Extra-thick retaining rings provide the necessary clearance for the spanner wrench to secure the lens without coming into contact with the optic surface.