1.1 Advantages of Polymer Optical Fibers
Polymer Optical Fibers (POF) offer many advantages
other data communication solutions such as glass fibers, copper cables
and wireless communication systems.
In comparison to glass fibers POF offer easy and cost-efficient
processing and are more flexible for plug packing. POF can be passed
with smaller radius of curvature and without any disruption because of
larger diameter in comparison with glass fiber.
The one clear advantage of using glass fibers is the low attenuation,
which is below 0,5db/km near the infrared range. In comparison, POF can
only provide acceptable attenuation in the visible spectrum from 350nm
up to 750nm, see fig. 1. The attenuation has its minimum with about
85db/km at approximately 570nm. For this reason POF can only be
efficiently used for short distance communication. The disadvantage of
the larger core diameter is higher mode dispersion.
Copper as communication medium is technically outdated, but still the
standard for short distance communication. In comparison POF offers
lower weight and space. Another reason is the nonexistent
susceptibility to any kind of electromagnetic interference [1-3
Wireless communication is afflicted with two main disadvantages. The
electromagnetic fields can disturb each other and probably other
electronic devices. Additionally, wireless communication technologies
provide almost no safeguards against unwarranted eavesdropping by third
parties, which makes these technologies unsuitable for the secure
transmission of volatile and sensitive business information.
For these reasons POF is already applied in various applications
sectors. Two of these fields are described in more detail: the
automotive sector and the in house communication sector.
1.2 POF in the automotive
Principle and Attenuation of POF in the visible range [1
POF displaces copper in the passenger compartment for
applications. It was first introduced by BMW in the so-called 7er
series in 2001. Since then not only high class cars were equipped with
POF, even volume cars benefit of the advantages of POF [4,5
The exchange of the communication medium delivers lower weight. For
example, in the Mercedes S-Class the weight was reduced by about 50kg
due to the exchange of the transmission medium used.
The glass temperature of POF (lower 100°C) makes using the
fiber in the engine compartment impossible [4
although this problem might be solved in the foreseeable future.
Another application in the car, where POF most likely will be used in
the future, is as sensors for measuring various in-car pressures or
1.3 POF in the in house communication sector
Another sector where POF displaces the traditional communication medium
is in-house communication [6,7], although the possibilities of
application are not confined to the inside of the house itself. In the
future POF will most likely displace copper cables for the so-called
last mile between the last distribution box of the telecommunication
company and the end-consumer. Today, copper cables are the most
significant bottleneck for high-speed internet.
“Tiple Play”, the combination of VoIP, IPTV and the
classical internet, is introduced in the market forcefully and
therefore high-speed connections are essential. It is highly expensive
to realize any VDSL system using copper components, thus the future
will be FTTH.
As mentioned before, POF can be applied in the house itself for
- “A/V Server Network” (communication
between e.g. television, hi-fi-receiver and DVD-player)
- “Control Server Network” (messaging
between e.g. refrigerator and stove)
- “Data Server Network” (data exchange
between e.g. notebook and printer)
These different application layers are further illustrated in fig. 2.
1.4 The Motivation for
WDM over POF
In the last two preceding paragraphs several sectors where shown, where
POF offers advantages in comparison to the established technologies.
Other possible industrial sectors include the aerospace or the medical
sector. But all these applications have one thing in common –
they all need high-speed communication.
The standard communication over POF uses only one single channel [1,2
To increase bandwidth for this technology the only possibility is to
increase the data rate, which lowers the signal-to-noise ratio and
therefore can only be improved in small limitations.
This paper presents a possibility to open up this communications
bottleneck. In glass fiber technology, the use of the WDM (wavelength
division multiplexing) in the infrared range at about 1550nm has long
been established practice [8-10
This multiplexing technology uses multiple wavelengths to carry
information over a single fiber [11
This basic concept can – in theory – also be
POF. However POF shows different attenuation behavior, see fig. 1. For
this reason, only the visible spectrum can be applied when using POF
For WDM two key-elements are indispensable, a multiplexer and a
demultiplexer. The Multiplexer is placed before the single fiber to
integrate every wavelength to a single waveguide. The second element,
the demultiplexer, is placed behind the fiber to regain every discrete
wavelength. Therefore the polychromatic light must be split in its
monochromatic parts to regain the information. These two components are
well known for infrared telecom systems, but must be developed
completely new, because of the different transmission windows.
Several technical solutions for this problem are available, but none of
them can be efficiently utilized in the POF application scenario
described here, mostly because these solutions are all afflicted with
high costs and therefore not applicable for any mass production [12
Fig. 2 In house Communication with POF
2. Basic Concept of the
In the last preceding paragraph it is shown, that a
open the bottleneck of bandwidth of standard POF communication is to
adapt WDM for the visible wavelength range. Therefore newly designed
multiplexers and demultiplexers are essential. Fig. 3 shows the sketch
of the basic concept of a demultiplexer for POF communication [13-15
]. The basic idea is to use
a prism to separate the different wavelengths.
The light emerges the standard POF, characterized by a core diameter of
980µm and a cladding-thickness of 10µm. The core
of PMMA (Polymethyl methacrylate) with a refractive index of 1.49. The
numerical aperture has the value 0.5. Hence the light emerges under an
aperture angle of 30° [1
The divergent light beam must be focused and separated to spot on the
detection layer. For focusing the light a convex lens is applied in the
basic concept. The separating of the different colors (wavelengths) is
done by a prism with low reciprocal dispersive power. In the principle
sketch only three colors were drawn. In the simulations only three
colors, blue (480nm), green (530nm) and red (660nm), were used as well.
This is not a limitation for possible future developments, but rather
an experimental basis from where to run the various simulations
Fig.3 Principle sketch of the patented basic concept
2.1 The Focusing Element
The light spotting on the detection layer is being
into electrical signals. To regain a good signal quality the different
channels must be focused on this layer. For this reason a convex lens
is applied. Lenses are always afflicted with certain aberrations.
Especially the chromatic and the spherical aberrations can avoid
unitary spot size. The stronger the curvature the stronger the
spherical aberrations, see fig. 4.
Fig 4. Spherical Aberration for different lens forms: a) simple
biconvex lens, b) lens “best form”, c) distribution
refraction power in two lenses, d) aspheric, almost plano convex lens 
The figure shows the
different lens forms. A biconvex lens demonstrates the strongest
spherical aberrations. A unitary focus point, not only for gauss beam,
can be achieved with a plano convex lens. Hence the first simulations
are run using this particular lens form to focus the light on the
detection layer to suppress aberrations.
2.2 The Dispersion Element
The different transmitted wavelengths are separated by means of a
prism. A prism refracts light in different directions in dependence on
the refractive index and should have a high dispersive performance. In
other words a prism should have a low Abbe-number. The lower the
Abbe-number the better is the separation of the different colors and
the broader is the gap between every single color on the detection
layer. The refractive index performance in dependence on the visible
range is shown in fig. 5 for several optical materials.
Fig. 5 Refractive index in dependence of wavelength
The steeper the curve
falls the larger is the gap between the colors.
3. Results of the
the following step, a software program is used to design a
demultiplexer based on the general concept outlined above. For the
current task, the software OpTaLiX provides all needed functionalities .
This approach offers different advantages, it is easy to design,
analyze and evaluate the simulated results. Also effective improvements
of the configuration can be simulated fast.
3.1 Early Results of the
The basis of the design for the demultiplexer is the
concept as shown in fig. 3. The 2D Plot of one of the first simulation
attempts is illustrated in fig. 6.
The single biconvex lens is displaced by two plano convex lenses. By
means of these two lenses spherical aberrations are minimized. The
first lens, consisting of layers 1 and 2, is placed before the high
dispersive prism, layer 3 and 4. This lens collimates the divergent
light beam. It can be calculated that the spherical and chromatic
aberrations are minimized if collimated light will hit the prism. The
second lens, layer 5 and 6, is situated behind the prism to focus the
light on the detection layer, layer 7.
To analyze this configuration the spot diagram in the detection layer
is examined. A spot diagram collects the transverse aberrations in the
image plane (detection layer) resulting from tracing a rectangular grid
of rays (emerging from a single object point) through the system.
The spot diagram of this configuration is shown in fig 6. A shift of
the different focus points along the optical axis is visible. The green
focus point is only visible on the detection layer. The blue focus
point is in front of the detection layer and the red focus point is
behind the detection layer. Hence the focus points overlap each other.
For that reason it is not possible to detect all of the three colors.
The prime reason of this shift along the optical axis is the spherical
aberration which is caused by the two lenses and the prism. To lower
this effect improvements have to achieve to detect every color without
Fig. 6 2D Plot and Spot Diagram of early simulation
Improved Results of the Simulation
The configuration in the last preceding paragraph cannot
separate all three colors. Hence improvements are essential to meet
satisfying results. Several steps lead to the configuration shown in
Fig. 7 2D Plot and Spot Diagram of the improved results
One main difference is
displacement of the first lens. The collimating of the divergent light
beam is effected by means of an off-axis parabolic mirror, layer 1.
This step was necessary to eliminate the spherical aberrations of the
first lens. Therefore this mirror is placed that the focus point of the
parabolic mirror is identical with the focus point of the emerging
light beam of the POF: In this configuration the mirror collimates the
light perfectly without any aberrations.
Another change is the extension of the optical path between the lens
and the detection layer. The longer this path the lower is the
spherical aberration of the second lens. This is founded is in the
lower curvature of the lens, layers 4 and 5, also see fig. 4.
The spot diagram of the improved configuration shows the three
channels, blue, green and red, which are separated completely. The gap
between every single color is about 5mm in length. For that reason
crosstalk is absolutely negligible. Hence this configuration satisfies
all the requirements of a demultiplexer for WDM over POF.
3.3 Verification of the
The simulated results must be verified by a first
dimensions of the elements, the off-axis parabolic mirror, the prism
and the lens, of the improved configuration are too small for optics
available on market. Therefore a slightly different configuration is
designed with elements available on market, see fig. 8.
Fig. 8 2D Plot and Spot Diagram of the verification design
In comparison to the
configuration the results of the assembly are satisfying, see fig. 9.
It is visible that all three channels can be separated. Because of the
extremely complex adjustment of the components, the results of the
assembly are not 100% congruent in comparison to the simulated results.
Reasons are the higher lens aberrations than expected and
lateral/longitudinal mechanical uncertainties in the first laboratory
Fig. 9 Prototype and Spot Diagram
3.4 Injection Molding
The demultiplexer should
by means of the injection molding technology. This technology is a very
inexpensive way to produce a high amount of devices with low costs.
In injection molding it is possible to process polymer materials. The
improved configuration is simulated with PC (polycarbonate) for the
prism and PMMA for the lens. The POF consists of POF as well. Hence the
complete configuration can be produced by means of injection molding.
In this manufacturing technique molten plastic is injected at high
pressure into a mold, which is the inverse of the product's shape. The
mold is made by a moldmaker (or toolmaker) from metal, usually either
steel or aluminum, and precision-machined to form the features of the
desired part [18
]. To get
first results of
this technique to assure that it is possible to fabricate optics, a
first waveguide has been manufactured, fig 10.
Fig. 10 Sketch
and Photo of
first injection molded device
core of the fabricated waveguide consists of PMMA and the cladding is
made of PC. The average value of the attenuation of the fabricated
waveguides at different wavelengths used for the demultiplexer is
measured and is shown in table 1.
|Table 1 Attenuation of the
molded device at different wavelengths
The Polymer Optical
many advantages in comparison to glass fiber and copper as the medium
for communication. The mentioned applications show different sectors
where POF is already applied.
State of the art for POF communication is the use of only one single
channel. This means a limitation of bandwidth. The solution for this
bottleneck is WDM over POF, there not only one channel is used to
transmit information over a single fiber. To use this technique two key
elements have to be designed a multiplexer and a demultiplexer.
The simulation results show, that it is possible to build up a
demultiplexer by means of a prism. The improved configuration can
separate the colors (channels) without any overlap. So in combination
with injection molding technology this configuration can be produced
with sufficiently costs. This demultiplexer has the chance to break
through the limitation of standard POF communication.
The verification of the simulated results by a first assembly shows the
correctness of the simulated results.
The next step will be the assembly of the optics made of injection
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