Traditional optical fibers address many applications well, but sometimes they are a case of having a round peg to fill a square hole.
Franz Schuberts, Axel Hoben, Kevin Bakhshpour and Chery l Provost, CeramOptec
A square-core optical fiber makes a better match with laser diode output beams, allowing
greater coupling efficiency. Square-core fibers also offer advantages over circular
fibers in applications such as spectroscopy and laser machining, where rectilinear
illumination patterns are needed.
Although the circular cross section of a traditional optical fiber
offers many useful attributes, there are many applications where a different geometry
would serve better. The near-field output beam of a laser diode, for example, typically
has a 10:1 oblong shape with a greater divergence along the short axis than on the
long axis. This beam does not match well with a traditional optical fiber unless
the core diameter is much larger than the beam’s long axis. The circular output
beam of a fiber also creates challenges when generating sharp corners and rectangular
shapes during laser surface treatments of materials.
Figure 1. Optical fibers with a square
core (second from top) are commercially available with traditional round cladding
as well as with square cladding. Courtesy of CeramOptec.
It is easier to manufacture optical fibers having a circular cross
section, but there is no fundamental reason that fiber cores cannot have other shapes,
including octagonal, rectangular and square, and CeramOptec now offers all of these
alternative core shapes. Fibers with a square core can have a circular cladding
(Figure 1) so that they are compatible with standard ferrules and mountings, or
they can have a uniform cladding to maintain the square shape. Numerical apertures
between 0.2 and 0.37 are typical.
Uniform output intensity
As might be expected, square fibers are highly multimode and perform
a great deal of ray mixing during optical propagation. The result of this mixing,
however, is homogeneous output beam intensity across the core area. This nearly
ideal top-hat intensity profile (Figure 2) is particularly useful in laser machining
and surface treatment applications. Imaging the square core onto the target surface
readily produces rectangular treatment areas with sharp corners and edges1, with
minimal cost. Creating a square treatment beam from a circular fiber output beam
requires masking – which reduces the usable beam energy – as well as
a complex and costly optical design.
Figure 2. The output beam of a square fiber exhibits homogeneous power density across the
emitting surface. Courtesy of CeramOptec.
Besides having a square near-field output beam, the square fiber
has a noncircular acceptance region. The result is a greater coupling efficiency
between the fiber and a laser diode. The typical laser diode produces an output
beam with initial dimensions of approximately 1 x 100 μm. Further, along its
wide-dimension beam, the divergence is 5° to 10°, while across the long
dimension, the divergence is 25° or more. A round fiber can capture this rapidly
diverging beam effectively near the fiber’s center, but the capture percentage
declines quickly with distance for the off-center beam. A square fiber can capture
the beam with equal efficiency along the length of the diode’s beam.
The square-core fiber with square cladding offers several advantages
when used as part of a fiber bundle in applications such as spectroscopy. In these
cases, the fiber bundle is packed together at one end to maximize the capture of
the light, then arranged so the fibers form a line at the bundle’s other end
to maximize the illumination of the spectroscope’s entry slit (Figure 3).
At the capture end, the square fiber array can provide up to 25 percent greater
energy capture by eliminating gaps found in circular fiber arrays. At the output
end, the square fibers provide a more uniform slit illumination than is possible
with circular fibers.
Figure 3. Square optical fibers have greater capture area in bundles than round fibers
and can provide a more uniform linear illumination pattern for spectroscopic applications.
Courtesy of CeramOptec.
Square fibers are particularly useful in astronomical spectroscopy
applications, which have several unusual requirements. In an astronomical spectroscope,
the telescope images the star field onto a fiber bundle, with the result that each
fiber typically captures the light from a different star. The rearranged bundle
at the spectrograph end aligns with the input slit, but because there is sufficient
space left between fibers, the instrument produces a column of individual spectrum
from each fiber. Because the mapping from the image end to the spectrograph end
is known, the spectrograph can analyze all the individual objects in the image field
Optical fibers used in astronomical applications must meet many
requirements.2 One is high transmission for the optical spectrum of interest. This
is a function of the core and cladding material choices, not shape, and is not significantly
different between square and circular fibers.
A second requirement is low focal ratio degradation. The ideal
fiber in an astronomical spectroscope would preserve the f number of the imaging
optics when delivering light to the spectroscope. In practice, however, the fiber
output has a higher f number than that of the imaging optics, and the amount of
focal ratio degradation increases with f number. An optical fiber receiving an f/8
image from the telescope, for instance, might deliver only an f/16 image to the
spectrograph. This focal ratio degradation complicates the spectroscope’s
optical design as well as decreases the amount of light available for forming the
A third requirement for astronomical optical fibers is a high
degree of image scrambling. Scrambling prevents the optical fiber from carrying
any information on the star image’s radial position at the fiber input. If
position information were preserved, it would affect the optical geometry inside
the spectrograph, and a shift in the star’s radial position on the fiber (such
as from atmospheric refraction) would result in a shift of the spectrum’s
position on the CCD sensor (Figure 4). Because the spectrograph uses a CCD array
to sense the spectral pattern, with each pixel in a row collecting energy for a
specific wavelength, a change in the spectrum’s position on the CCD would
place optical energy into the wrong bins.
Figure 4. Insufficient image scrambling in round optical fibers can cause shifting in the
spectrum’s position within a spectrograph as the star image moves across the
fiber surface, decreasing the instrument’s effective wavelength resolution.
Courtesy of Gerardo Avila, European Southern Observatory.
As with focal ratio degradation, the amount of image scrambling
a fiber exhibits increases with increasing f number. This forces instrument developers
into a trade-off. The faster (more light gathering) the optical design, the less
the focal ratio degradation but the worse the spectral variations resulting from
Square fibers eliminate trade-offs
Early results from some CeramOptec customers indicate that square
fibers exhibit less focal ratio degradation than corresponding circular fibers.
In addition, the square fibers show much greater scrambling than circular fiber,
so that, at low f numbers, the spectrograph is less affected by seeing conditions
and guidance errors that alter a star’s radial position on the fiber. These
improved attributes eliminate the need for a trade-off between focal ratio degradation
and scrambling in astronomical spectroscope design. Coupled with the greater capture
area of a square fiber bundle, the focal ratio degradation and scrambling improvements
make square fibers a much better match to astronomical applications than traditional
Figure 5. Although optical fibers with circular cross sections are
easy to manufacture, there is no fundamental reason why fiber cores cannot have
other shapes, including octagonal. Courtesy of CeramOptec.
The shape of the square fiber core thus provides better solutions
than the traditional circular fiber to a variety of application challenges. Increased
capture area in bundles, efficient match to noncircular beams and uniform output
power distribution have significant benefits in spectroscopy, laser surface treatments
and diode laser coupling. The rectangular near-field beam shape also can be of benefit
in any application that requires uniform rectilinear illumination.
Meet the authors
Franz Schuberts is sales manager of industrial and scientific
fiber optics at CeramOptec GmbH in Bonn, Germany; Axel Hoben is a sales engineer
for industrial and scientific fiber optics at CeramOptec GmbH; Kevin Bakhshpour
is vice president of sales and marketing at CeramOptec Industries Inc. in East Longmeadow,
Mass.; and Cheryl Provost is a scientific/industrial sales engineer at CeramOptec
Industries Inc.; e-mail: firstname.lastname@example.org.
1. J.R. Hayes et al (Oct. 30, 2006). Square core jacketed air-clad
fiber. J Opt Soc Amer, Vol. 14, No. 22, pp. 10345-10350.
2. Samuel C. Barden (1998). Review of fiber-optic properties for
astronomical spectroscopy. Astronomical Society of the Pacific Conference Series,
Vol. 152, pp. 14-19.