Using multimode interference in visible spectrum transmission
Allen Jong-Woei Whang 1, 2, Kuan-Yu Chen 1 and Way-Ne Chen 2
1
2
Photonics System Simulation and Design Laboratory, Department of Electronic Engineering
Photonics System Simulation and Design Laboratory, Department of Electro-Optical Engineering
National Taiwan University of Science and Technology
#43, Sec. 4, Keelung Rd., Taipei, 106, Taiwan, R.O.C
ABSTRACT
A multimode interference device is usually used in a variety of optical communication components, is a mature
development of integrated optical components. Traditionally, the original of integrated optics usually be used in the field
of optical communications, we will extend the applications of multimode interference device in this paper, we try
multimode interference device applications in the field of natural light illumination systems and the lighting guides
which our goal is each visible wavelength of light coupled with a waveguide transmission.
In our proposed, we use poly(dimethylsiloxane)(PDMS) to fabricate this device, there is a high transmission
efficiency(>95%) and over a very large bandwidth. The device demonstrated good confinement of visible light and low
attenuation at 0.40 dB/cm.
Keywords: multimode interference, natural light illumination systems, polymer waveguide, beam propagation method
1
INTRODUCTION
1.1
Background
Under the influence of global warming, green energy and low-carbon life, the question more and more widely noted that
in 1997, the United Nations signed the Kyoto Protocol. Our goal is still in the stabilization of greenhouse gases in the
atmosphere for an appropriate level and then to avoid drastic climate change, the damage caused by humans. Green
energy is into the spotlight. Pollution caused by the Earth's environment will not be repeated, reusable energy, such as
solar energy, is the key. The pursuit of extreme and efficient use of renewable energy is a hot topic. Solar, wind,
hydropower, ocean energy, geothermal energy and biomass are renewable energy. In renewable energy research, the
most important issue is how to use energy efficient. In renewable energy, solar energy has many advantages, an
abundance of natural and clean. In addition, it is a global energy, so we can use it anywhere in the world. This article
how to collect the sun's interior lighting is our ultimate goal.
1.2
Motive
Major reasons for many natural light illumination systems are high capital cost. And there is a lot of cost about the
transmission, in traditional are using optical fibers or reflection light pipe. There are expensive manufacture costs of
those two elements. Therefore we proposed using the MMI structure to guide the visible spectrum and makes those could
be used in long distance transmission and indoor illumination.
2
BASIC THEORY AND PRINCIPLES
2.1
System structure
Saving energy and reducing carbon dioxide are among the most critical global issues. Therefore, solar energy should be
used as much as possible. One method is photovoltaic generation to convert sunlight directly into electricity
[1,2]
.
However, low efficiency and high cost are the drawbacks. In addition, there is a system which is able to gather natural
light for indoor lighting. It can save 80 percent of the energy used by traditional lighting
[3,4]
. This is the Natural Light
Guide System. In this study, we mainly design a new light pipe with optical structures in the light transmitter system.
The Natural Light Guiding System consists of three parts：the light collector, the light transmitter, and the light emitter,
shown in Fig1. In the light collector part, using the sunlego to collect sunlight, the theorem about sunlego use is to collect
sunlight and use total reflection to effectively compress the light by prism total internal reflection. That is to change the
surface light source into a linear light source or point source, shown in Fig2. The element in the transmitting part is
optical fiber. Optical fiber can transmit the collected light to our wish place with the least loss. Although the optical fiber
will increase the cost, in this time fiber is the first choice. The optical fiber we would like to design is placed in this part,
as shown in the Fig. 3.
Fig. 1 A schematic diagram of the Natural Light Guiding System.
Fig. 2 The light collector of the natural Light Guiding System.
Fig. 3 Figure the circle in part connections with fiber.
2.2
Multimode interference device
Self-imaging of periodic objects illuminated by coherent light was first described more than 170 years ago
[5]
. The
multimode waveguide region is designed so that its length coincides with one of the self-imaging planes at its output, so
that, for example, each input waveguide is imaged onto one output waveguide. This self-imaging phenomenon, which
arises in multimode optical waveguides, can be used to image a small spot size beam into the middle of a micro channel.
Shows in fig.4 single images of the input field, the length of MMI section should be designed to fulfill the following
condition:
Lmmi  m( Lx )  n(3Ly )
where m and n are odd integers. In the paired and general excitations [6] are assumed to apply to the x and y dimensions,
respectively.
Fig. 4 The single images of the input field
3
Multimode interference device of visible spectrum
3.1
Material
Polydimethylsiloxane (PDMS) is one of the most widely used flexible silicone elastomers. It is an ultra-violet transparent,
gas permeable silicon-based, organic, cross-linkable polymer that can withstand a wide temperature range. Due to its
numerous outstanding material properties PDMS is applied in more and more fields in science and industry: e.g. it is
very common in biology-related experiments and in clinical practice due to its low toxicity or biocompatibility, but as
replication molds or stamps it is also frequent in the field of nano or micromachining technology. PDMS is often used to
create microfluidic devices, such as separation systems, micro-mixers or even diagnostic chips e.g. for DNA analysis.
Due to the high optical clarity, low attenuation and the excellent chemical stability PDMS is great choice to create optical
components. Since this polymer is a good replicating material the optical structures (waveguides, microlenses, etc.) are
usually created by amolding process, in which PDMS is first coated on to and then peeled off from a rigid mold, or with
imprinting technology. The PDMS waveguide demonstrated good confinement of light and relatively low attenuation at
0.40 dB/cm. also has very high light transmittance ( 95%) over a large frequency range.
3.2
Simulation and design
The size of waveguide was fixed at 20μm width and a change in fixed-width waveguide length for different
wavelengths. There are different length in visible spectrum that red light 1990μm, green light 2550μm and blue light in
the PDMS waveguide. Three wavelength self-imaging point by using the least common multiple of the calculation, the
main point of the RGB wavelength with the position at148 cm.
Fig.5 The green light distribution of energy propagation
Fig.6 The red light distribution of energy propagation
Fig.7 The blue light distribution of energy propagation
4
CONCLUSION
After we proposed the concept of using multimode interference in visible spectrum Provides a new approach about the
natural light illuminations system in indoor illumine. In the future We hope to be able to improve the transmission of
different wavelengths caused by the imaging point of difference, through the structure and choice of materials, making
the imaging point more gathered together in a different wavelength of visible light spectrum.
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