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OPTICS / LENSES / Spherical Bi-Concave Lenses
Diverging Lenses (Negative Lenses): Thinner in the middle, they cause parallel light rays to spread out, making them appear to emanate from a virtual focal point.
Biconcave Lenses: Both surfaces are concave with the same radius of curvature.
Applications include: Focusing Light From a Point Source or from a Parallel Beam; Collimating Diverging Light;
Image Formation: used as objective lenses components;
Beam Expansion/Reduction; Spatial Filtering to remove higher spatial frequencies (noise);
Fiber Optics Coupling focus a free-space laser beam onto the small core of an optical fiber or collimate light emerging from a fiber.
Bi-Concave lenses have a negative focal length and are best used to diverge a converging beam. Similar to a Plano-Concave lens, a Bi-Concave lens can diverge a collimated beam to a virtual focus as used in a Galilean type beam expander. But it is usually preferrable to choose a bi-concave lens if the absolute conjugate ratio (object distance divided by image distance) is close to 1. When the desired absolute magnification is either less than 0.2 or greater than 5, the tendency is to choose a plano-concave lens instead.
4.1) Laser Line Bi-Concave Lenses (DAR Coated)
POL-BK7-BCV-DAR785-12.7 Series
Laser line bi-concave lenses are fundamental tools in photonics research, providing essential capabilities for manipulating laser beams, correcting optical imperfections, and enabling advanced experimental techniques. Their diverging nature makes them indispensable for beam expansion, light sheet generation, and integration into complex optical systems.
For laser applications, bi-concave lenses often feature specialized anti-reflection coatings optimized for specific laser wavelengths. These coatings minimize light loss due to reflection at the lens surfaces, maximizing transmission and preventing unwanted interference or damage from high-power lasers.
4.2) Bi-Concave Lenses (Uncoated)
The most significant advantage of UVFS is its excellent transparency in the deep ultraviolet (UV) and visible spectral ranges, from approximately 185 nm to 2100 nm.
The high purity of UVFS, with a low content of microscopic defects and inclusions, gives it a very high laser-induced damage threshold. This allows these lenses to be used with high-energy and high-power lasers without the risk of material damage, ensuring long-term reliability.
UVFS has a very uniform refractive index throughout the material. This exceptional homogeneity ensures minimal wavefront distortion, which is vital for maintaining the quality and profile of a laser beam as it passes through the lens.
4.3) DIA12.7mm BK7 Bi-Concave Lenses AR@600-1100nm
BK7 (or N-BK7, a lead-free version) is a very common type of borosilicate crown glass. It’s a popular choice for commercial optics due to its Excellent optical properties, Broad transmission range, Good physical properties: It is relatively hard and resistant to scratches, and it’s chemically stable.
An anti-reflection (AR) coating is a thin film applied to the lens surfaces to minimize light loss due to reflection. Whenever light passes from one medium to another (like from air into glass), a portion of it is reflected at the interface. This can lead to reduced system efficiency, “ghost” images, and even damage in high-power laser systems.
4.4) DIA25.4mm BK7 Bi-Concave Lenses DAR@400-700nm
BK7 (or N-BK7, a lead-free version) is a very common type of borosilicate crown glass. It’s a popular choice for commercial optics due to its Excellent optical properties, Broad transmission range, Good physical properties: It is relatively hard and resistant to scratches, and it’s chemically stable.
V-Coats vs. Broadband AR:
A V-coat is an AR coating optimized for a very narrow wavelength range, typically a specific laser line (e.g., 1064 nm for a Nd:YAG laser). The plot of reflectance versus wavelength is V-shaped, with the minimum at the design wavelength.
A broadband AR (BBAR) coating is a multi-layer design that reduces reflections over a wider range of wavelengths within the NIR spectrum.
4.5) DIA25.4mm BK7 Bi-Concave Lenses AR@600-1100nm
BK7 (or N-BK7, a lead-free version) is a very common type of borosilicate crown glass. It’s a popular choice for commercial optics due to its Excellent optical properties, Broad transmission range, Good physical properties: It is relatively hard and resistant to scratches, and it’s chemically stable.
V-Coats vs. Broadband AR:
A V-coat is an AR coating optimized for a very narrow wavelength range, typically a specific laser line (e.g., 1064 nm for a Nd:YAG laser). The plot of reflectance versus wavelength is V-shaped, with the minimum at the design wavelength.
A broadband AR (BBAR) coating is a multi-layer design that reduces reflections over a wider range of wavelengths within the NIR spectrum.
4.6) Bi-Concave Lenses DAR@1510-1610nm
Optical Fiber Communication: The 1550nm band (which includes 1510 nm to1610 nm) is a critical window for long-haul optical communications because optical fibers have minimal signal loss at these wavelengths. Lenses with DAR coatings in this range are used to couple lasers into fiber optics, or to collimate light coming out of a fiber.
Lasers operating at 1510 nm to1610 nm can be combined for specific applications like telecommunications (where these wavelengths are critical for optical fiber communication) or medical research. The coating ensures minimal loss for both beams.
4.7) BK7 Bi-Concave Lenses (SAR MgF2)
An anti-reflection (AR) coating of Magnesium Fluoride (MgF2) has been applied to reduce light reflection and increase transmission. MgF2 is a very common and cost-effective single-layer AR coating
The primary purpose of the MgF2 coating is to minimize the amount of light that is reflected away from the lens surfaces. This is based on the principle of destructive interference, where the thin film of MgF2 cancels out the reflected light waves. A typical uncoated lens can reflect around 4% of the light per surface, but an MgF2 coating can reduce this to less than 1.5% over a wide wavelength range.
When deposited correctly, MgF2 coatings are hard and durable, making the lens more resistant to scratches and environmental damage. This durability is important for lenses used in harsh environments or those that require frequent handling and cleaning