HarzOptics GmbH

WDM over POF – The inexpensive way to break through the limitation of bandwidth of standard POF communication

U.H.P. Fischer (member IEEE)
Hochschule Harz, Friedrichstraße 57-59, 38855 Wernigerode

Matthias Haupt
Hochschule Harz, Friedrichstraße 57-59, 38855 Wernigerode

Abstract: Polymer Optical Fibers (POF) are used in various fields of applications, e.g. in the automotive industry or the in house communication technology. Applications in these fields require increasingly more bandwidth, therefore developers become tasked with finding new solutions to increase the technical efficiency of all communications equipment. One solution is wavelength division multiplexing (WDM). WDM allows the transmission of information over more than just a single wavelength (color) and thus greatly increases the POF’s bandwidth. Different wavelengths which are jointly transmitted over the fiber must be separated to regain all information. These separators are called Demultiplexers. There are several systems available on the market, which are all afflicted with certain disadvantages. The most common and grave disadvantage almost all of these systems exhibit is their costly production, which makes them unsuitable for today’s price sensitive mass markets. Hence the goal of this paper is to develop an inexpensive Demultiplexer for WDM transmission over POF. The fundamental idea is to separate the chromatic light in its monochromatic components with the help of a prism with low reciprocal dispersive power. The prism and the other assemblies which are needed to adjust the optical path could be manufactured in injection molding technology. This manufacturing technique is a very simple and cost-efficient way to produce a Demultiplexer for POF.

1 Introduction

1.1 Advantages of Polymer Optical Fibers

Polymer Optical Fibers (POF) offer many advantages compared to 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.

Principle and Attenuation of POF in the visible range
Fig.1 Principle and Attenuation of POF in the visible range [1]
1.2 POF in the automotive sector

POF displaces copper in the passenger compartment for multimedia 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 forces.

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 different scenarios:
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 assigned to POF. However POF shows different attenuation behavior, see fig. 1. For this reason, only the visible spectrum can be applied when using POF for communication.

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].

In house Communication with POF

Fig. 2 In house Communication with POF

2. Basic Concept of the Demultiplexer

In the last preceding paragraph it is shown, that a solution to 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 consists 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 described below.

Principle sketch of the patented basic concept

Fig.3 Principle sketch of the patented basic concept

2.1 The Focusing Element

The light spotting on the detection layer is being converted 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.

Spherical Aberration for different lens forms: a) simple biconvex lens, b) lens “best form”, c) distribution of refraction power in two lenses, d) aspheric, almost plano convex lens

Fig 4. Spherical Aberration for different lens forms: a) simple biconvex lens, b) lens “best form”, c) distribution of refraction power in two lenses, d) aspheric, almost plano convex lens [16]

The figure shows the behavior of 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.

Refractive index in dependence of wavelength

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 Simulation

In 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 [17]. 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 Simulation

The basis of the design for the demultiplexer is the general 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 overlap.

2D Plot and Spot Diagram of early simulation

2D Plot and Spot Diagram of early simulation
Fig. 6 2D Plot and Spot Diagram of early simulation

3.2 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

2D Plot and Spot Diagram of the improved results
Fig. 7 2D Plot and Spot Diagram of the improved results

One main difference is the 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 Simulated Results

The simulated results must be verified by a first assembly. The 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.

2D Plot and Spot Diagram of the verification design

2D Plot and Spot Diagram of the verification design

Fig. 8 2D Plot and Spot Diagram of the verification design

In comparison to the simulated 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 set-up.

Prototype and Spot Diagram

Prototype and Spot Diagram

Fig. 9 Prototype and Spot Diagram

3.4 Injection Molding

The demultiplexer should be produced 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.

Sketch and Photo of first injection molded device

Sketch and Photo of first injection molded device

Fig. 10 Sketch and Photo of first injection molded device

The 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.

Attenuation [dB] /per piece
66nm (red) 530nm (green) 480nm (blue)
1,80 2,17 2,52

Table 1 Attenuation of the molded device at different wavelengths

4 Conclusion

The Polymer Optical Fiber features 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 molding.


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