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What are Ball lenses and Half-ball lenses:
Ball lenses belong to a special form of biconvex lenses which have the geometry of a ball(sphere). They are manufactured from a single material, usually an optical glass with good transparency in the wavelength region of interest. The typical applications of ball lenses include focusing light in the field of fibers(e.g. laser to fiber coupling ,fiber to fiber coupling), emitters and detectors , majorly to collimate light depending on geometries of the input light source. Also they could be ball pre-forms of aspheric lenses where they are purposefully deformed in order to prevent spherical aberrations.
Half-Ball lenses are variants of ball lenses, obtained by simply cutting the ball lenses in half.Due to ease of mounting brought by the one flat surface,they are ideal for applications where more compact designs are required , such as fiber communication, endoscopy, microscopy, optical pick-up devices, and laser measurement systems.
There are three essential parameters of ball lenses an half-ball lenses.One is effective focal length (EFL),which is the distance between a plane through the center of the lens and the beam waist (focus) of an initially collimated input beam.Another is back focal length (BFL), defined as the distance of the focal point from the lens surface, therefore half the diameter smaller than the EFL.The calculation equations of EFL and BFL are given below. And the last is numerical aperture (NA) ,for collimated incident light, the numerical aperture (NA) of the ball lens is dependent on the size of the ball lens (D), its index of refraction (n), and the diameter of the input source (d). Simply put, numerical aperture is proportionate to the resolution of the lens, the larger the NA,the more the light collected by the lens. And the equation is also given below.
Hangzhou Shalom EO provides both customized and stocked ball lenses and half ball lenses made from Sapphire, a portfolio of ball lenses and half ball lenses made from various materious including N-BK7, Fused Silica and Sapphire are also available .The specification of custom ball and half ball lenses could be varied upon your request.
There are some typical features of N-BK7, UV fused Silica and Sapphire:
N-BK7: N-BK7
is a very pureand S
tandard glass types used to produce high-quality optics, mainly for the visible spectrum.
It is one of the most frequently used optical glass,
Th
is
relatively hard
and stable
borosilicate crown material
possesses high purity and quality with
very few inclusions and is almost free of bubbles.The high quality
of
material
of
lN-BK7 glass is a primary technical reason why
it
is
the optimized
choice for optical
applications
.
Other features of N-BK7 which are advantageous for optical usage include
smooth transmission
and
low absorption in the entire visible wavelength range
.It is worth noting that under normal conditions
N-BK7
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has a more instable polishing and is softer than FS,
and additionally
N-BK7 has higher losses through higher absorption and scattering and therefore
are relatively less suitable
for high power applications.
UV Grade Fused Silica:
Sapphire: Optcial grade Sapphire&#;chemically composed as aluminum oxide (Al2O3) with mono crystalline structure, is used in transmission range from 0.2 - 5.5μm,which is a fairly wide range, and is particularly suitable for MWIR 3-5μmthermal imaging applications.
Sapphire belongs to the trigonal crystal system and has a hexagonal structure.The lattice constant is a=b=4.758A, c=12.991A,Refractive index 1.762-1.770, Birefringence: 0.008~0.010.The melting temperature of sapphire is extremely high , °C,which enables sapphire to be engaged in manifolds of specially applied working conditions,high temperature applications, and applications requiring high melting points.
Sapphire lenses are made from single crystal sapphire, they are ideal for high demanding applications because of their outstanding performance, consisting of superior surface hardness ( 9 on the Mohs scale ,the third hardest mineral, after diamond at 10 and moissanite at 9.5, which means high resistance to scratch and abrasion ), high thermal conductivity, high dielectric constant and resistance to common chemical acids and alkalis. Additionally, sapphire features a high index of refraction and excellent broadband transmission characteristics.
Tags: Ball Lenses and Half-Ball Lenses
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Author: the photonics expert Dr. Rüdiger Paschotta
A special form of a thick biconvex optical lens is a ball lens, usually having the geometrical form of a ball (sphere). They are manufactured from a single material, usually an optical glass with good transparency in the wavelength region of interest. A frequently used material is fused silica.
Another variant are half-ball lenses, which are obtained simply by cutting ball lenses in half.
Ball lenses are usually made with relatively small diameters of a few millimeters or sometimes even less than 1 mm (microlenses). Particularly for such small dimensions, they are easier to fabricate than lenses with traditional designs.
Figure 1:
Focusing of light with a ball lens. While the paraxial rays have a focus position as indicated with the gray vertical line, the outer rays are more strongly refracted.Figure 2:
Focusing of divergent light with a ball lens.Ball lenses exhibit substantial spherical aberrations when light propagation is not restricted to a small fraction of its cross-section. Examples are shown in Figures 1 and 2.
A special kind of micro-ball lens is obtained by heating the end of a tapered fiber such that it melts.
Ball lenses are used particularly as beam collimators for optical fibers (fiber collimators) and for fiber-to-fiber coupling. They are also suitable for miniature optics with applications like barcode scanning, as objective lenses in endoscopy and for optical sensors. There are also microscope objectives (particularly immersion objectives) which have a hyperhemisphere (e.g., somewhat more than a hemisphere) as the first lens.
There are two different definitions of focal length of a ball lens. The effective focal length, which is the distance between a plane through the center of the lens and the beam waist (focus) of an initially collimated input beam, is given by the equation
$$f = \frac{{n\;D}}{{4(n - 1)}}$$$$f = \frac{{n\;D}}{{4(n - 1)}}$$
where <$D$> is the diameter of the lens ball and <$n$> its refractive index.
The back focal length is defined as the distance of the focal point from the lens surface, and is smaller than the effective focal length by half the diameter of the ball.
Just as other spherical lenses, ball lenses exhibit optical aberrations and in particular spherical aberrations (see Figure 1 and 2) when operated with incident beams having a diameter which is not much smaller than that of the ball. Therefore, the minimum possible spot size of the focus is not obtained for the largest possible input beam size, as it would be for a perfect lens.
It is possible to produce aspheric lenses with much weaker aberrations, using spherical ball lenses as a preform which are then appropriately deformed.
Natural ball lenses in the form of small water droplets cause the phenomenon of rainbows. The color effects of the primary (most prominent) row arise from light paths with a single internal reflection in a droplet. Sometimes one can see a secondary rainbow, arising from beam paths with two internal reflections.
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