SPECTRAL GRIDS FOR VIS WDM APPLICATIONS OVER SI-POF

SPECTRAL GRIDS FOR VIS WDM APPLICATIONS OVER SI-POF
                                        M. Joncic, M. Haupt, U. H. P. Fischer
   Photonic Communications Lab, Harz University of Applied Sciences, Friedrichstr. 57-59, 38855 Wernigerode, Germany.
                                     Corresponding author: mjoncic@hs-harz.de

Abstract: After exploiting the capabilities of a single channel, the next step to increase the capacity of an indi-
vidual POF is to use multiple channels over a single fiber what is well known as WDM. In this paper possible
approaches to designing a spectral grid in visible spectrum are presented. In addition, different frequency and
wavelength domain spectral grids for visible spectrum WDM applications over SI-POF are presented.
Key words: Standard step index polymer optical fiber, wavelength division multiplexing, spectral grids, visible
spectrum applications.

1. Introduction

1.1. Motivation for WDM over POF
Standard communication systems over Polymer Optical Fibers (POFs) use a single channel (single wavelength)
for data transmission. Strong modal dispersion, caused by high numerical aperture of 0.5, severely limits the
bandwidth - length product of standard step-index POF (SI-POF) to approximately 50MHz ⋅ 100 m [1]. This
certainly allows 100 Mbit/s transmission, but for higher data rates spectrally efficient modulation schemes must
be used. Working groups over the world make strong efforts to fully utilize the available bandwidth of SI-POF.
The usable frequency range is determined not only by 3-dB optical bandwidth but also by signal-to-noise ratio of
a channel [2]. Although the transmission capacity of the channel is greatly increased, the compromise between
complexity of a system and its market potential must be made.
POF, with its numerous advantages as a transmission medium, has a strong potential to replace traditional com-
munication mediums in consumer’s part of access network [3]. However, due to constantly growing bandwidth
demands, a capacity offered by a single channel may present a bottleneck of such a system. After exploiting the
capabilities of a single channel, the next step to increase the capacity of an individual POF is to use multiple
channels over a single fiber what is well known as wavelength division multiplexing (WDM). In glass fiber
technology the use of WDM in the infrared (IR) range is well established. In fact, WDM is a key technology in
modern optical long-haul communications, allowing hundreds of optical channels to be transferred over a single-
mode glass optical fiber and thus providing virtually unlimited capacity.
For multi-Gbit/s transmission over a single 100 m SI-POF WDM technology will inevitably have to be de-
ployed. Combining independent bandwidth-efficient channels onto a single SI-POF by means of WDM technol-
ogy will make SI-POF an ultimate transmission medium for short-range communication systems. This gives a
strong motivation not only to invest time and resources to develop a low-cost and efficient demultiplexer for SI-
POF systems [4,5,6], but also to make efforts to standardize spectral grid for future WDM systems.

1.2. Motivation for spectral grid in VIS
The only alternative to a spectral grid for WDM applications in the visible spectral range is to declare individual
channels that are independent from each other. For a system with two or three wavelengths this is a reasonable
solution. However, for a system with more than three wavelengths this would result in a solution that would
certainly have much poorer performances than a grid. In order to fully utilize the transmission spectrum, and thus
maximize performances and capacity of a possible system, spectral grid must be used.
Following the good practice established through ITU-T G.694.1 and G.694.2 recommendations, which provide
frequency and wavelength grids for applications in IR part of the spectrum [7,8], the goal is to establish spectral
grid in visible spectrum (VIS). When defining a grid, ITU-T principles and terminology should be followed.
Each proposed grid should be characterised by nominal central frequencies (wavelengths), channel spacing, and
optionally, anchor frequency (wavelength). Characterization of a grid does not stop there, and further discussion
will be given in the next section.

2. Approaches to designing a grid
Approach to designing a spectral grid includes defining criteria and conditions that have to be fulfilled in order

to achieve desired performances. Generally speaking, a spectral grid in VIS can be generated in two ways: (a) by
expanding existing grids from IR into visible spectral range; (b) by generating independent wavelength and fre-
quency grids. In the first case an approach is needed to evaluate the performances of the existing grid. In the
second case an approach is needed to generate a grid or to evaluate previously generated grid. Before a discus-
sion about possible approaches some remarks are made.
When designing a spectral grid in VIS, advantage should be given to extensions of the existing grids from IR
into visible spectral range. If an extension itself satisfies specified requests, and a system with desired perfor-
mances that can use it can be designed, standardization process would be much easier since bond between visible
and IR spectral range would exist. Only in a case when extensions do not show good performances, independent
grids should be designed.
Independent grids can be designed in both frequency and wavelength domain. Conversion of a grid with uni-
formly distributed nominal central frequencies from frequency into wavelength domain results in non-uniform
distribution of nominal central wavelengths. Channel spacing is smaller in lower wavelength region and rises at
higher wavelengths. In POF communication systems characteristics of elements (e.g. optical sources, filters) are
given in wavelength domain. Consequently, it is easier to describe a grid in wavelength domain. If a grid is
mathematically defined in frequency domain and then converted and described in wavelength domain, require-
ments for different channels would differ (e.g. allowed spectral width of a source). Even though having a grid in
frequency domain is not dismissed as a solution, in order to avoid previously described situation and have uni-
form design of a grid in wavelength domain, future work will be concentrated on the grids defined and described
in wavelength domain.
The grid in VIS should be designed to work with light emitting diodes (LEDs). Unlike laser diodes (LD), LEDs
are available within the whole spectral range in which SI-POFs are used. Furthermore, usage of LEDs imposes
much stricter requirements for a spectral grid. If a grid is designed for LEDs, the system will work when LED is
substituted with LD in one or even all channels. Vice versa would not be possible. Finally, demands for a WDM
system in VIS are modest – a several channels. In the calculations LEDs with a Gaussian spectral distribution
were used.

2.1. Approach based on the shape of spectral attenuation curve
The first approach to developing a grid is based on the shape of spectral attenuation curve of standard SI-POF
(see Fig. 1). The basic idea is to mathematically describe a grid so that nominal central wavelengths (frequen-
cies) are placed at wavelengths or at least wavelength regions where attenuation curve has its minimums. The
wavelengths where attenuation curve has local minimums are 522 nm (green), 568 nm (yellow), and 650 nm
(red). Attenuation curve does not have very big variations around 568 nm, and especially around 522 nm. On the
other hand, the attenuation rapidly increases for wavelengths around 650 nm. Therefore, spectral grid should
contain a nominal central wavelength (frequency) at exactly 650 nm (461.2 THz), and channel spacing that will
place channels at, or at least close to 568 nm (527.8 THz) and 522 nm (574.3 THz).

                                   Fig. 1: Spectral attenuation of standard SI-POF.

With this approach, a grid defined by nominal central wavelengths, channel spacing and reference wavelength
(optional) can easily be generated. However, the approach is incomplete – the system that can be used with a
grid still has to be described. The procedure of designing such a system represents a part of the following sepa-
rate approach.

2.2. Approach based on characterizing the transmitter side
The second approach is far more complex. It includes characterization of elements on the transmitter side (opti-
cal sources and filters) and spectral analysis. But first a structure of a WDM system on the transmitter side will
be considered. In this consideration a multiplexer is assumed to be a black box that simply combines spectrums
of input signals onto the output fiber. Two possible design solutions are to: (a) use filter to limit the spectrum of
a LED before spectrums are combined; (b) simply combine unaffected LED spectrums onto a single fiber. The
second approach is based on a solution where filters are used to limit the output spectrum of LEDs. An overview
of this approach is presented even though all the requirements and parameter values are still not completely
defined.
The main idea on which the second approach is based is to maximize the number of channels in VIS but still
avoid crosstalk between adjacent channels after they are brought together. This can be achieved only by limiting
the output spectrum of LEDs with filters. At this point it is not important whether filtering is performed separate-
ly in each channel or by a multiplexer itself.
LED can be roughly characterized by its central wavelength and spectral width. In case of spectral grids in VIS it
can be quite ungrateful to declare one wavelength as a central wavelength of a channel. It is rather possible to
declare a wavelength region within which all wavelengths are mutually equal. This wavelength region presents a
region of central wavelengths for that channel. One of these wavelengths (most likely the central wavelength of
the region) can certainly serve as a reference for that channel. By introducing a certain tolerance for central
wavelength the possibility to have available LED in some particular wavelength region is increased, and it is
possible to minimize the effective attenuation of a channel.
The next step after defining a spectral region of central wavelengths is to take into account tolerances for central
wavelength due to the fabrication process. A certain drift of central wavelength can certainly be expected. Tem-
perature effects on the output spectrum of the LED should also be taken into account (temperature coefficient of
central wavelength and temperature coefficient of the output power). The system should work at temperatures
between 0° C and 70° C. In order to minimize the temperature drift of a central wavelength, the system should be
designed at 35° C.
Transfer function of the filter should be designed in a way that allows sufficient optical power at the output of
the filter for any of the previously defined operating conditions. In order to design a realistic transfer function,
the capabilities of multimode filters in the visible spectrum should be considered. Variation of filter’s central
wavelength should also be taken into account.
At this point a virtual channel is completely described. Term virtual is used because no wavelength is still as-
signed. In order to perform presented procedure a decision about the values of the following parameters has to be
made: width of a central wavelength region, spectral width of a LED, central wavelength drift due to the fabrica-
tion process, temperature coefficient of central wavelength, temperature coefficient of output power, maximal
insertion loss of a filter, variation of filter’s central wavelength. Some compromises between these values will
certainly have to be made.
A virtual WDM system is created by placing virtual channels next to each other. The following step is to closer
virtual channels to the distance that will still keep the crosstalk at the acceptable level. Once this is performed,
the position of a virtual WDM system on the wavelength axis should be determined. This is a step after which
there is: (a) a spectral grid defined by nominal central wavelengths, channel spacing and reference wavelength
(optional); (b) completely characterized transmitter’s side of a WDM system that can be used with a grid. The
final step is to calculate effective attenuation in each channel for appropriate range of wavelengths. To determine
optical power, which is necessary for reliable transmission at certain distance in each channel, a whole WDM
system should be characterized.
In comparison to the combining of unaffected LED spectrums, the advantage of this approach lies in a fact that
utilization of the available spectrum is much better. In order to have acceptable crosstalk after combining unaf-
fected LED spectrums, the channel spacing should be very big. The complete analysis in this case is still to be
performed.

2.3. Choosing an approach
The advantage of the first approach is that it gives a good insight into the shape of spectral attenuation curve.
Attenuation minimums can be identified and hopefully channels can be placed at those positions. However, the
flexibility when designing a WDM system that can be used with a grid can be dramatically reduced, resulting in
poorer performances of a system. The supported opinion is that the second approach should be the basic ap-
proach. Once a WDM system is designed, the first approach can be used to position it on wavelength axis. After
having an insight into its possible positions on wavelength axis, changes can be made through the second ap-

proach in order to position it in the best possible way.

3. Frequency domain grids

3.1. DWDM grid extension
ITU-T G.694.1 recommendation provides a frequency grid for dense wavelength division multiplexing
(DWDM) applications. Term dense refers to narrow optical frequency spacing. The frequency grid, anchored to
193.1 THz (1552.52 nm), specifies channel spacing of 100 GHz (0.8 nm), 50 GHz (0.4 nm), 25 GHz (0.2 nm),
and 12.5 GHz (0.1 nm). The grid extends through L, C, and S-bands covering the frequency range from 186 THz
(1611.79 nm) to 201 THz (1490,50 nm).
Standard SI-POFs are used for data transmission in the visible part of the optical spectrum, i.e. between
428.3 THz (700 nm) and 749.5 THz (400 nm). Extending DWDM frequency grid into the visible spectral range
results in 3213 channels between 428.3 THz and 749.5 THz for the channel spacing of 100 GHz. In wavelength
domain channel spacing reduces from 0.163 nm for channels at 700 nm region, to 0.053 nm for channels at
400 nm region. For channel spacing of (100/n) GHz, where n=2,4, or 8, number of channels in the same spectral
region is n*3212+1.
Demands for a WDM system, and thus for a grid, are far more modest in visible than in IR spectral range. A
frequency grid in VIS defined on basis of extended DWDM grid can only take over an anchor frequency, which
has nothing in common with VIS, and just few nominal central frequencies. Defining an extension of a DWDM
grid that would expand over whole VIS, and then using up to 10-15 frequencies to define VIS WDM grid does
not represent a reasonable solution. Furthermore, channel spacing in VIS, must be far bigger than ITU-T recom-
mendation specifies. This is another element where touching point between DWDM grid extension and grid
defined on its basis is lost.
Each grid defined with nominal central frequencies that belong to extended DWDM grid can be considered to
belong to DWDM grid extension. All frequency grids that have been defined in VIS have for anchor frequency
and channel spacing a whole number or number with one decimal. This means that any frequency grid defined in
VIS can most likely be linked to extended DWDM grid.
Taking into account all mentioned above, DWDM grid extension is discarded as a solution for generating a grid
in VIS. Independent grids should rather be defined in frequency domain.

3.2. Independent frequency grids
Independent frequency grid presented here is anchored to 461.2 THz (650 nm), and has a channel spacing of
22.2 THz. It is generated on the basis of the spectral attenuation curve approach. Allowed nominal center fre-
quencies (in THz) are defined by fi = 461.2+i*22.2, where i=0,1,2,…,13. Grid consists of 14 frequencies posi-
tioned within the range from 461.2 THz to 749.8 THz, i.e. 14 wavelengths within the range from 650 nm to 400
nm. Fig. 2 shows this grid in wavelength domain. This grid completely satisfies the requirements specified by
spectral attenuation curve approach – it has nominal central frequencies at 461.2 THz (650 nm), 527.8 THz (568
nm), and 572.2 THz (524 nm).

                Fig. 2: Frequency grid anchored to 461.2 THz (650 nm) with channel spacing of 22.2 THz.

Constant channel spacing in frequency domain results in variable channel spacing in wavelength domain. Here
channel spacing varies from 12 nm for two channels at lowest wavelengths, to 30 nm for two channels at highest
wavelengths. Due to small channel spacing, especially in the lower wavelength region, further analysis of this

network would not end up with positive result. There was an idea to divide this grid in two sub-grids, but it was
abandoned fast. Anyway, this is a nice example how to mathematically define a grid based on the shape of the
attenuation curve.
All other attempts to define a grid in frequency domain that satisfies requirements specified by spectral attenua-
tion curve approach were not as successful. Therefore, no other frequency grids are presented here.

4. Wavelength domain grids

4.1. CWDM grid extension
ITU-T G.694.2 recommendation provides a wavelength grid for coarse wavelength division multiplexing
(CWDM) applications. Term coarse refers to wider channel spacing. Recommendation specifies channel spacing
of 20 nm. CWDM grid consists of 18 wavelengths defined within the range from 1271 nm to 1611 nm.
Extending CWDM frequency grid into the VIS results in 15 wavelengths within the range from 400 nm to
700 nm, as shown in Fig. 3. The channels that are positioned within the lowest attenuation regions are those at
651 nm, 571 nm, and 511 nm. Having a channel at 651 nm is particularly important. Fig. 4 shows normalized
Gaussian spectrums of three LEDs with spectral width (FWHM) of 20 nm. Their spectrums are centered within
the adjacent channels of CWDM grid extension, i.e. at 631 nm, 651 nm, and 671 nm. A transfer function of an
ideal bandpass filter (transmission in passband 100%, transmission in stopband 0%) that has a passband of
20 nm is also shown in Fig. 4.

                                Fig. 3: CWDM grid extension into the visible spectrum.

        Fig. 4: Normalized Gaussian spectrums of 3 LEDs (631 nm, 651 nm, 671 nm) in CWDM grid extension and
                                       transfer function of ideal bandpass filter.

The ideal bandpass filter introduces insertion loss (IL) of 1.2 dB. If the width of the passband is reduced to
16 nm, IL rises to 1.85 dB. If additionally a 3 nm drift of LED’s central wavelength is introduced, IL equals to
1.93 dB. Central wavelength drift of 3 nm does not cause big additional loss in this case. However, if the trans-
mission in passband is reduced to 90%, IL rises to 2.4 dB. Described situation is far from realistic, but it shows
how strict requirements for LEDs and filters a channel spacing of 20 nm will impose. A system that can use this
grid is still to be designed, but it is questionable how realistic it will be. Even though a CWDM grid extension is

not dismissed as a solution, an optimal solution at this point is to design an independent wavelength grid.

4.2. Independent wavelength grids
The analysis performed up to now indicates that initial efforts in designing a spectral grid in VIS should be fo-
cused on designing an independent grid in wavelength domain. Due to the complexity of the designing process,
complete analysis of possible solutions in wavelength domain is still not available. Instead, some grids generated
on the basis of spectral attenuation curve approach are presented.
The first grid, anchored to 650 nm, has a channel spacing of 27 nm. It consists of 11 channels placed between
407 nm and 677 nm. Grid satisfies requirements of the approach since it places channels at 650 nm, 568 nm, and
514 nm. The second grid, anchored to 650 nm, has a channel spacing of 30 nm. It consists of 10 channels placed
between 410 nm and 680 nm. It does not place the channels on wavelength axis as good as the first one, but still
has a channel at 650 nm, 560 nm, 530 nm, and 500 nm. The last presented grid, also anchored to 650 nm, has a
channel spacing of 35 nm. It consists of 9 channels placed between 405 nm and 685 nm, including channels at
650 nm, 580 nm, and 510 nm.

5. Conclusions
The first step towards having a functional spectral grid in VIS is to define an approach to designing it. In this
paper possible approaches to designing a spectral grid in VIS are identified and described. Furthermore, a choice
of the approach that is going to be used is made. Extensions of DWDM and CWDM grids into the VIS are pre-
sented and analyzed. Independent spectral grids in frequency and wavelength domain are also presented. Defined
approach to designing a spectral grid in VIS is still to be fully implemented.
WDM over SI-POF is a technology that will significantly increase the capacity of SI-POF. Therefore, strong
efforts are made to develop low-cost and efficient WDM components in VIS. Parallel to that, initial efforts are
made in the standardization sector. Based on already established spectral grids in IR range, the goal is to have a
standardized spectral grid in VIS.

Acknowledgements
This publication is based on a project funded by the German Federal Ministry of Education and Research (pro-
motional reference 16V0009 (HS Harz) /16V0010 (TU Bs)). The author is responsible for the content of this
publication.

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