design of optical fiber 50/50 y coupler & 60/40 y coupler
TRANSCRIPT
California State University, Northridge
Design of Optical Fiber 50/50 Y Coupler & 60/40 Y Coupler & Their Use Cases
A graduate project submitted in partial fulfillment of the requirements
For the degree of Master of Science in Electrical Engineering
By
Neha Jayeshkumar Chauhan
May 2018
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The graduate project of Neha Jayeshkumar Chauhan is approved:
_________________________________________ __________
Dr. Xiaojun Geng Date
_________________________________________ __________
Dr. Jack Ou Date
_________________________________________ __________
Dr. Nagwa E. Bekir, Chair Date
California State University, Northridge
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Acknowledgement
“Design of Optical Fiber 50/50 Y Coupler & 60/40 Y Coupler & Their Use Cases” is my graduate
project submitted in partial fulfillment of the requirements for the degree of Master of Science in
Electrical Engineering. It took two semesters of hard work to complete the project. I wish to
express my gratitude to Dr. Nagwa Bekir, Committee chair, for her time, support, suggestions and
invaluable guidance that helped me make this project a success. I would also like to thank Dr.
Xiaojun Geng and Dr. Jack Ou for agreeing to serve in my committee and reviewing my project.
I thank my parents and family, without whom my quest for a graduate degree would have remained
a dream. Finally, this project has been a motivation for me to carry on research work in the field
of Fiber Optics.
Neha Jayeshkumar Chauhan
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Table of Contents
Signature Page ii
Acknowledgement iii
List of Figures vi
Abstract ix
Chapter 1: Introduction 1
Chapter 2: Optical Coupler Types and Applications 3
2.1 Classification of fiber optic couplers 3
2.1.1 Passive fiber optic couplers 3
2.1.2 Active fiber optic couplers 4
2.2 Fiber optic coupler types 5
2.2.1 Y coupler 6
2.2.2 T coupler 6
2.2.3 1xN or Tree coupler 7
2.2.4 Star coupler 7
2.2.5 Wavelength division multiplexing (WDM) or Wavelength
selective coupler 9
2.2.6 Directional coupler 10
2.2.7 Fiber optic combiner 10
2.2.8 X-coupler 11
2.2.9 Acoustic coupler 11
2.3 Applications of fiber optic couplers 12
Chapter 3: Mathematical Model of Optical Y Coupler 14
3.1 Theory 14
3.2 Consequence of conservation of energy 16
3.3 Delay normalization 16
3.4 Y coupler unitary matrix 16
Chapter 4: Introduction of BPM, OptiBPM, Design Process & Simulation of 50/50 Y
Coupler and 60/40 Y Coupler 18
4.1 Beam Propagation Method (BPM) 18
4.2 OptiBPM 18
4.2.1 Numerical simulations 19
4.2.2 2D BPM 19
4.2.3 Scan parameters automatically 19
4.2.4 Mode solvers 19
4.2.5 Graphics 19
4.2.6 Introduction to optical waveguides 20
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4.3 Design process of 50/50 Y coupler 20
4.3.1 Drawing the output waveguides and viewing the simulation results
in OptiBPM analyzer 30
4.3.2 Generating data script 39
4.3.3 Exporting scattering data 43
4.4 Design process of 60/40 Y coupler 43
Chapter 5: Use Cases of Optical 50/50 Y Coupler and 60/40 Y Coupler with the
comparison of 1x2 Power Splitter by using OptiSystem 53
5.1 Use case 1: 50/50 Y coupler 53
5.2 Use case 2: 60/40 Y coupler 57
5.3 Use case 3: 50/50 Y coupler 58
5.4 Use case 4: 60/40 Y coupler 62
Conclusion & Summary 65
References 66
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List of Figures
Figure 2.1(a): Fiber optic coupler 3
Figure 2.1(b): Fiber optic coupler mechanism 3
Figure 2.1.1(a) Passive fiber optic coupler 4
Figure 2.1.1(b): Core/butt joint of optical fibers 4
Figure 2.1.1(c): Surface interaction of optical fibers 4
Figure 2.2: Fiber optic coupler types: (a) Three-port couplers- Splitter and Combiner (b) Four-
port coupler in general (c) Star coupler (d) Wavelength selecting or wavelength division
multiplexing/demultiplexing couplers 5
Figure 2.2.1: Y coupler 6
Figure 2.2.2a: T coupler 6
Figure 2.2.2b: T coupler network 6
Figure 2.2.3: Tree coupler or 1xN coupler and Nx1 coupler 7
Figure 2.2.4a: Star coupler 8
Figure 2.2.4b: Star coupler- Outputs are as the combination of inputs 8
Figure 2.2.4c: Fiber twisting, fusing or melting and tapering process 8
Figure 2.2.4d: Reflective/Non-Directional Star coupler 9
Figure 2.2.4e: Non- Directional Star coupler working 9
Figure 2.2.5: Wavelength selective/ WDM couplers 10
Figure 2.2.7: Fiber optic combiner 11
Figure 2.2.8: X coupler or 2x2 coupler 11
Figure 2.3.1: Star couplers are used in star network topology 12
Figure 2.3.2: T couplers are used in bus network topology 13
Figure 2.3.3: Tree couplers are used in cascaded PON architecture 13
Figure 3.1: Coupling light through a 2x2 fiber optic coupler 14
Figure 3.2: Coupling light through a 1x2 fiber optic coupler 14
Figure 4.3.1: Initial properties dialog box 21
Figure 4.3.2: Profile designer window 21
Figure 4.3.2a: Dielectric “guide” dialog box 21
Figure 4.3.2b: Cladding definition 22
Figure 4.3.2c: Defined channel profile 22
Figure 4.3.3: Initial properties dialog box - Default waveguide tab 22
Figure 4.3.3a: Initial properties dialog box - Wafer dimensions tab 23
Figure 4.3.3b: Initial properties dialog box - 2D wafer properties 23
Figure 4.3.4: Layout window 23
Figure 4.3.4a: Start offset values - First waveguide 24
Figure 4.3.4b: End offset values - First waveguide 24
Figure 4.3.4c: Start offset values - Second waveguide 25
Figure 4.3.4d: End offset values - Second waveguide 25
Figure 4.3.5: Basic coupler layout 25
Figure 4.3.6a: Input plane properties dialog box 26
Figure 4.3.6b: Item on input fields 2D tab 26
Figure 4.3.7: Modified basic structure of coupler 26
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Figure 4.3.8: Simulation parameters dialog box - 2D tab 27
Figure 4.3.9: Simulation - 2D optical field 27
Figure 4.3.9a: OptiBPM analyzer- Optical field propagation XZ slice 28
Figure 4.3.9b: Data clamping dialog box 28
Figure 4.3.9c: Simulation with new data clamping settings 28
Figure 4.3.9d: Region of interest dialog box 29
Figure 4.3.9e: Simulation - Region of interest 29
Figure 4.3.10: Modified second linear waveguide properties 30
Figure 4.3.10a: Modified wafer properties 30
Figure 4.3.10b: Modified layout 30
Figure 4.3.11: Start offset value - First output waveguide 31
Figure 4.3.11a: End offset value - First output waveguide 31
Figure 4.3.11b: Red waveguides 32
Figure 4.3.11c: Path monitoring type 32
Figure 4.3.12: Simulation with output waveguide 32
Figure 4.3.12a: Analyzer layout with output waveguide 33
Figure 4.3.12b: Analyzer - Optical field propagation XZ Slice with output waveguide 33
Figure 4.3.12c: Analyzer - Path monitor iteration 33
Figure 4.3.12d: Data clamped view with output waveguide 34
Figure 4.3.12e: Refractive index propagation XZ slice with output waveguide 34
Figure 4.3.13: Start offset value - Second output waveguide 35
Figure 4.3.13a: End offset value - Second output waveguide 35
Figure 4.3.14: Final settings of output waveguides - 50/50 optical Y coupler 36
Figure 4.3.14a: 50/50 Y coupler simulation result 36
Figure 4.3.14b: 3D view of 50/50 Y coupler 36
Figure 4.3.14c: 2D view of 50/50 Y coupler 37
Figure 4.3.14d: Path Monitor of 50/50 Y coupler 37
Figure 4.3.14e: Cut view of 50/50 Y coupler 37
Figure 4.3.15: Analyzer layout of 50/50 Y coupler 38
Figure 4.3.15a: Analyzer optical field propagation XZ slice of 50/50 Y coupler 38
Figure 4.3.15b: Analyzer - Path monitor iteration 38
Figure 4.3.15c: Data clamped view 39
Figure 4.3.15d: 3D view 39
Figure 4.3.15e: Refractive index propagation XZ slice 39
Figure 4.3.16: Scripting window- Scattering data script 40
Figure 4.3.16a: Simulation parameters dialog box 41
Figure 4.3.16b: Scripted 50/50 Y coupler simulation result 41
Figure 4.3.16c: Path monitor of scripted 50/50 Y coupler 42
Figure 4.3.16d: Analyzer optical field propagation XZ slice of scripted 50/50 Y Coupler 42
Figure 4.3.16e: Analyzer - Path monitor iteration 43
Figure 4.4.1: Initial properties dialog box - Default waveguide tab 44
Figure 4.4.1a: Initial properties dialog box - Wafer dimensions tab 44
Figure 4.4.2: Start offset values - First waveguide 44
Figure 4.4.2a: End offset values - First waveguide 45
Figure 4.4.2b: Start offset values - Second waveguide 45
Figure 4.4.2c: End offset values - Second waveguide 46
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Figure 4.4.2d: Start offset value - First output waveguide 46
Figure 4.4.2e: End offset value - First output waveguide 47
Figure 4.4.2f: Start offset value - Second output waveguide 47
Figure 4.4.2g: End offset value - Second output waveguide 48
Figure 4.4.3: Final settings of output waveguides - 60/40 optical Y coupler 48
Figure 4.4.4: Scripting window - Scattering data script 49
Figure 4.4.5: Scripted 60/40 Y coupler simulation result 50
Figure 4.4.5a: Cut view 50
Figure 4.4.5b: Path monitor 50
Figure 4.4.6: Analyzer layout of 60/40 Y coupler 51
Figure 4.4.6a: Analyzer optical field propagation XZ slice of 60/40 Y coupler 51
Figure 4.4.6b: Refractive index propagation XZ slice 51
Figure 4.4.6c: Analyzer - Path monitor iteration 52
Figure 5.1.1: Simple use case of 1x2 power splitter 53
Figure 5.1.1a: CW laser properties 54
Figure 5.1.1b: 1x2 power splitter properties 54
Figure 5.1.2: Calculation output 55
Figure 5.1.2a: Optical power meters reading 55
Figure 5.1.3: OptiBPM NxM component properties for 50/50 Y coupler 56
Figure 5.1.3a: Simple use case of 50/50 Y coupler 56
Figure 5.1.4: Optical power meters reading 57
Figure 5.2.1: 1x2 power splitter properties 57
Figure 5.2.1a: Optical power meters reading 57
Figure 5.2.2: OptiBPM NxM component properties for 60/40 Y coupler 58
Figure 5.2.2a: Simple use case of 60/40 Y coupler 58
Figure 5.2.2b: Optical power meters reading 58
Figure 5.3.1: 50/50 power splitter system 59
Figure 5.3.1a: Spatial CW laser properties 59
Figure 5.3.1b: Thin lens properties 59
Figure 5.3.1c: Spatial connector properties 60
Figure 5.3.1d: Parabolic index multimode fiber properties – Main 60
Figure 5.3.1e: Parabolic index multimode fiber properties - Fiber profile 60
Figure 5.3.1f: 1x2 power splitter properties - ‘5 5’ 60
Figure 5.3.1g: Spatial PIN photodiode properties 61
Figure 5.3.2: Optical power meters and electrical power meters reading 61
Figure 5.3.3: 50/50 Y coupler system 61
Figure 5.3.3a: Optical power meters and electrical power meters reading 62
Figure 5.4.1: 1x2 Power splitter properties - ‘6 4’ 62
Figure 5.4.1a: 60/40 power splitter system and its power meters readings 63
Figure 5.4.2: 60/40 Y coupler system 63
Figure 5.4.2a: Optical power meters and electrical power meters reading 64
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Abstract
Design of Optical Fiber 50/50 Y Coupler & 60/40 Y Coupler & Their Use Cases
By
Neha Jayeshkumar Chauhan
Master of Science in Electrical Engineering
The main purposes of this project are to introduce different types of optical fiber couplers with
their wide range of applications for establishing optical fiber networks, design the very important
optical fiber 50/50 Y coupler and 60/40 Y coupler and give its use cases for various fiber optic
network systems with the comparison of 1x2 power splitter. The mathematical model of optical Y
coupler is also discussed in detail. The introduction of Beam Propagation Method (BPM),
OptiBPM software and OptiSystem software are given. By using OptiBPM software, both the
couplers have been designed, scripted, simulated and exported to OptiSystem software. Finally, in
OpiSystem software, the use cases of both the couplers have been created and the simulation results
have been compared with 1x2 power splitter. The whole procedure from coupler design to its use
cases is explained step by step.
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Chapter 1
Introduction
In networking and telecommunication infrastructure industry, fiber optic is the major building
block whose characteristics of low attenuation and high bandwidth makes it ideal for gigabit
transmission and beyond [1]. An optical fiber transmits light between two ends of the fiber which
can be defined as a light channel having a core of transparent and flexible very pure glass strand
surrounded by a dielectric cladding. By establishing a refractive index greater than the cladding,
the optical signals are confined in the core. As light waves travel longer distances through a core,
they constantly bouncing from the cladding which does not absorb any light from the core. This
phenomenon termed as a principle of total internal reflection. Multimode fibers are the fibers
analyzed by the geometric optics and having a sizable core diameter [2].
This project will give the introduction of fiber optic coupler fundamentals, design techniques used
to create a fiber optic coupler and its simulation results. It will also include the classification of
optical fiber couplers, their different types, subtypes, technologies and their applications. The
coupling losses and system performance will also be discussed in detail.
Optical coupler is complicated in design compare to its electrical counterparts because the fact that
the optical signals are complex than electrical signals. The optical signal is comprised by the flow
of signal carriers, photons in this case, just like electric current, but it does not flow through the
receiver to the ground. Rather, it gets absorbed by a detector at the receiver side. When multiple
receivers are cascaded, the entire signal would be absorbed by the first receiver and the rest of the
receivers would not receive any signal past the first one. Thus, the signal must be divided between
the multiple parallel output optical ports with diminishing its magnitude [3].
A fiber optic coupler works as an optical device which has the capability of distributing and
transmitting light from a primary optical fiber into multiple subsidiary optical fibers or connecting
multiple fiber ends to allow the transmission of light signals in various paths. Also, it can combine
multiple input signals into one output signal and can split a single input signal into multiple output
signals. These abilities make an optical fiber coupler one of the important parts of optical fiber
system. Additionally, optical fiber coupler provides the service to increase the numerous terminal
connections which should be permitted in the advancement of optical fiber communication
networks [4]. Also, the fiber optic couplers have the number of benefits such that minor excess
loss, immense stability, dual operating window, extreme reliability and little polarization
dependent loss. Furthermore, they have high directivity and small insertion loss. These are the
reason of their increasing requirements for different applications in the fiber optic information
distribution systems consisting data buses, community antenna networks, telecommunication
access networks, Local Area Networks (LANs) and computer networks [5].
The several hundreds of dB/km attenuation which early optical fibers had, has been scaled down
to 0.2dB/km in modern optical fibers at 1550 nm wavelength. In the parallel development of fiber
optic waveguide, other fiber optic components also got the consideration, by which the optical
fiber communication system would constituted. Thus, the losses of fiber optic couplers should be
around 0.2 dB/ km, but the signal can be more attenuated by them compared to a connector or a
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splicer because of the fiber coupling with losses and the input signal segmentation amongst the
output ports. This remains crucial aspect of fiber optic communication system. The conditions of
the ideal fiber optic couplers are light waves among the branch fibers should be distributed without
any scattering loss or noise generation and they should be totally insensitive to factors including
light distribution between the modes of optical fiber and light polarization state. Passive optical
fiber couplers unfortunately do not exhibit all these properties in practice. The finite scattering loss
at fiber optic coupler restricts the connected several terminals or alternatively the network span.
Whereas the modal effects and the noise generation can create problems in the network
performance specifications. Thus, the performance of optical fiber network gets affected by the
characteristics of the couplers and because of this reason, usually optical couplers cannot be
considered as individual elements with known parameters in a network. There are certain tradeoffs
necessitated in their application by this factor. In a typical optical fiber coupler, the considerable
amount of losses is present and therefore the goal of this project is to design an optical fiber coupler
that reduce the losses and improve the performance [4].
Chapter 2 will give the introductory detail for the classification of optical fiber couplers, different
types, subtypes and technologies of all optical couplers and their principle of operations followed
by their applications.
While in Chapter 3, the mathematical model of the Y coupler, its characteristics and working will
be discussed theoretically.
Chapter 4 will show the complete design process and the detailed simulation of the optical 50/50
Y coupler and 60/40 Y coupler by using OptiBPM software.
Finally, Chapter 5 will show the simulation results by comparing with the ideal 1x2 power splitter
and system performance will be analyzed in detail with the use of OptiSystem software. The
conclusion will be given by considering all the factors discussed in the project.
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Chapter 2
Optical Coupler Types and Applications
This chapter has included the classification of all optical fiber couplers with its various types and
technologies. Also, their principles of operations and applications are explained.
Fiber optic couplers can be called splitters when they used to divide the input signals into multiple
output signals, while they can be called combiners when used to connect two or more input signals
into one single output signal. As shown in Figure 2.1a and 2.1b, two input signals A and B are
coming from two different ports 1 and 2 respectively to optical coupler and the output signals
received at the other end of the coupler on ports 3 and 4, are the mix of both signals A and B [4].
Figure 2.1a: Fiber optic coupler
Figure 2.1b: Fiber optic coupler mechanism
2.1 Classification of fiber optic couplers
Optical fiber couplers can be of two primary types, either passive or active.
2.1.1 Passive fiber optic couplers: They do not require power to perform the operations. To
redirect the light signals, these classic components are used. To combine the core of the optical
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fibers together, passive couplers either use splice or optical mixers, beam splitters, micro-lenses
and graded-refractive-index (GRIN) rods.
Figure 2.1.1a: Passive fiber optic coupler
The power transfer in this type of couplers takes place by using one of these ways explained below:
(a) Core interaction type: by using butt joining technique or imaging optics between two
optical fibers through the optical fiber core cross section. Figure 2.1.1b shows this type.
Figure 2.1.1b: Core/butt joint of optical fibers
(b) Surface interaction type: the guided core modes get converted to the modes of refraction
and cladding to transfer power through the surface of optical fiber and make it normal to
its axis. This enables the mechanism of power sharing. Figure 2.1.1c displays this type [4].
Figure 2.1.1c: Surface interaction of optical fibers
2.1.2 Active fiber optic couplers: An external power source is required by them. By using optical-
to-electrical converters, optical fiber detectors and light sources for input and output, they work as
electronic devices that electrically combine or divide the signals [6].
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2.2 Fiber optic coupler types:
Optical coupler, optical splitter and optical combiner are all devices belonging to optical couplers.
Optical splitters are usually Y couplers, T couplers or tree couplers that have only single input port
and multiple output ports, and on contrary, optical combiners have only one output port and two
or more input ports. Figure 2.2 shows the list of different coupler types.
Figure 2.2: Fiber optic coupler types: (a) Three-port couplers- Splitter and Combiner (b) Four-port
coupler in general (c) Star coupler (d) Wavelength selecting or wavelength division
multiplexing/demultiplexing couplers
The following four main categories of Multiport couplers are differently configured:
• Three port-port devices- Y and T couplers- take input signal and split it between two outputs.
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• 1-to-n or Tree couplers- split one input among multiple outputs.
• Star couplers- Like star, optical fibers with a central mixing element radiate in outward directions.
• Wavelength-selective couplers- According to the wavelengths, distribute signals [5].
2.2.1 Y coupler
It simply splits the input signal into two output signals and known as Tap coupler. It can distribute
power equally, so that the each of two outputs receives half of the power transmitted. Also, the
ratio of power distribution can be controlled precisely between two output ports, such as in terms
of percentage, 10/90, 20/80, 30/70, 40/60 or 50/50 [5].
Figure 2.2.1: Y coupler
2.2.2 T coupler
The functioning of T coupler is same as Y coupler.
Figure 2.2.2a: T coupler
To connect multiple terminals on a single fiber optic network, T couplers can be cascaded that can
be seen in Figure 2.2.2b. The distribution of power in T couplers is uneven, so that all the output
signals carry the same incoming signal, but the power at one output is large compared to the second
output. It is necessary to have 10/90 or 20/80 split ratio between two outputs in percentage to
transfer enough power to the next terminal in the network link [5].
Figure 2.2.2b: T coupler network
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2.2.3 1xN or Tree coupler
It takes one or two input signals and divide them into multiple output signals and that is why tree
coupler is also known as 1xN coupler. By using this coupler, the input power is evenly distributed
among the output optical fibers. Also, it combines multiple input signals to one or more output
signals which works as a combiner. This can be seen in Figure 2.2.3.
1×4, 1×8, 1×16, 1×32 and 2×4, 2×8, 2×16, 2×32 port ratios are the most common configurations.
The main applications of tree couplers in LANs (Local Area Networks), CATV (Community
Antenna Television), and all other types of optical communication systems, where they are used
to divide and combine optical signals [6].
Figure 2.2.3: Tree coupler or 1xN coupler and Nx1 coupler
2.2.4 Star coupler
It has multiple input and output ports and both the ports could be same in number such as 2×2,
4×4, 8×8, etc. Like tree coupler, in this coupler also the input power is evenly distributed among
the output optical fibers. It can be divided in two subtypes based on directivity as below:
(a) Directional Star coupler
After mixing all the input optical signals, it distributes them among all outputs, as shown
in Figure 2.2.4a and Figure 2.2.4b. Also, it can be noted that directional star coupler
distributes an optical signal to all output ports introduced by any input port. It can transmit
light waves in the opposite direction, so works as a bidirectional device too.
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Figure 2.2.4a: Star coupler
Figure 2.2.4b: Star coupler- Outputs are as the combination of inputs
As shown in Figure 2.24c, the directional star coupler is made by the three different
processes- twisting, fusing or meting and tampering, on multiple optical fibers together.
Figure 2.2.4c: Fiber twisting, fusing or melting and tapering process
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(b) Non-Directional/Reflective Star coupler
Unlike the directional star coupler, it takes input light signals from all optical fibers and
distributes them among all input and output fibers, so as shown in Figure 2.2.4d and 2.2.4e,
light signal going in one of the optical fiber ports and emerges from all output ports,
including input port. It can have made by having a mirror at the end and reflecting the input
light signal into all ports and it is also called reflective star coupler [6].
Figure 2.2.4d: Reflective/Non-Directional Star coupler
Figure 2.2.4e: Non- Directional Star coupler working
2.2.5. Wavelength division multiplexing (WDM) or Wavelength selective coupler
Wavelength selective coupler or Wavelength division multiplexing/demultiplexing coupler is a
specialized form of coupler. It is designed to allow various optical signals of peak wavelength to
be transmitted parallelly on only optical fiber. It either multiplex i.e. mix the input optical signals
of various wavelengths onto the fiber or demultiplex i.e. the output optical signals of distinct
wavelengths get separated from the fiber according to the wavelengths, such as the light get
pumped from the amplified signal of an optical amplifier. Though it looks like 1x2 T coupler, it
does not only divide the power but also divide the distinct wavelengths into two outputs, for
example, 980/1550nm and 1310/1550nm. Also, wavelengths entering from the wrong output port
should get blocked by it [7].
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Figure 2.2.5: Wavelength selective/ WDM couplers
2.2.6 Directional coupler
This electronic coupler has four port circuits with one isolated port from the input port and one
through port. It is generally used to divide the input signals and distributed power. The device
couples part of the transmission power by a specific factor through one port. Directional couplers
are used in a wide range of applications which involve measurement, power monitoring and other
utilities.
Directional couplers are categorized as passive reciprocal networks. A directional coupler is used
for isolating, eliminating or combining signals in microwave signal routing and radio frequency.
The ports in the directional coupler are:
• Coupled
• Input
• Transmitted
• Isolated
A special design is put into use by which the input power is divide between the coupled and output
ports in a specific ratio known as the coupling ratio. Depending on the application for which it is
used, the key specifications of the directional coupler varies. The parameters/specifications which
are mostly varied are the coupling factor, transmission loss, low variation of the coupling
attenuation, high directivity and input power. For most directional couplers, the features which are
desired are high directivity, good impedance and wide operational bandwidth. But the performance
of a directional coupler is computed using the directivity factor. There are different types of
directional couplers like single, dual directional, coaxial, waveguide and even combination types
[2].
2.2.7. Fiber optic combiner
It receives two input signals and provides a single output signal as shown in Figure 2.2.7. The
attempt to combine two input signals of same wavelength creates the significant amount of
interference and due to this, the output signal is normally comprised of multiple wavelengths.
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Figure 2.2.7: Fiber optic combiner
2.2.8. X-coupler
It is a 2x2 coupler with the function of a combiner and a splitter, which is used to combine the
power of two input signals and splits the combined signal between the two output ports.
Figure 2.2.8: X coupler or 2x2 coupler
2.2.9 Acoustic coupler
It is an audio interface device for coupling a computer with audio into or out of a telephone. It may
also be a terminal device linking data terminals and radios with telephone networks. The link or
interface is done by picking up audio signals from a telephone handset rather than a direct electrical
connection.
Acoustic couplers were not allowed on telephones in the U.S. prior to 1982. Telephones were hard-
wired into the wall. Bell Systems often owned the telephones themselves. The telephone system
was a closed system entirely owned by Bell. However, elsewhere in the world acoustic couplers
were popular in the 1970s, but transmitted at speeds of only up to 300 baud – the number of voltage
fluctuations (frequency) on a telephone line. The practical upper limit of acoustic couplers was
1200 baud. These were made by Vedic in 1973 and by AT&T in 1977. However, modems replaced
acoustic couplers and could transmit data over phone lines more easily, dependably and at greater
transfer speeds. In the U.S. this happened rapidly after the breakup of Bell Systems in 1982. By
1985 this was widespread using the Hayes Smart modem 1200A, which allowed creation of dial-
up bulletin board systems – the precursor of today’s Internet chat rooms, message boards and
email.
Acoustic couplers were very sensitive to external noises. To fit closely to the telephone handset,
the attached cup had to be of a certain size. Therefore, the effectiveness of the device was
dependent on the standardization of the handset dimensions. Thus, when direct electrical
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connections were made legal in the U.S., modems became very popular, and acoustic couplers use
declined rapidly. However, some are still used by world travelers where electrical connections to
telephones are illegal or not available. And many models of telecommunications devices for the
deaf (TDD) still have acoustic couplers built-in, allowing universal use with pay phones [2].
2.3 Applications of fiber optic couplers
In local area network (LAN) applications, fiber optic couplers are used in either bus architecture
or star architecture.
As shown in Figure 2.3.1, in a star network topology, the central hub connected with the different
stations looks like the wheel having spokes in it, where each of these stations or network devices
are connected by the star couplers and can communicate with each other. To expand the numerous
work stations is easy with star couplers, for example, the system capacity doubles when changing
from a 4x4 to an 8x8 network.
Figure 2.3.1: Star couplers are used in star network topology
While in bus architecture, T couplers are used to connect number of stations in series to a single
backbone cable. The T coupler at each node of typical bus network topology divides the part of
the power from the bus and supplies it to the equipment attached.
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Figure 2.3.2: T couplers are used in bus network topology
T and Y couplers can be used quickly and easily for a small network set up in cascaded way and
at the terminals with different standard connectors for example, ST(Straight Tip Connector),
SC(Standard / Subscriber Connector), LC(Lucent Connector), FC(Ferrule / Fiber Channel
Connector), etc.
Tree coupler are typically used in cascaded PON architecture. The first tree coupler is directly
attached to the optical line terminal (OLT) port in the central office, then each of the output fibers
is routed to a tree coupler in other sites (outside enclosure/terminal box). If there’s needing to
further divide the signal, more tree couplers can be added. Of course, other amplification or
compensation modules are required to ensure the transmission.
Figure 2.3.3: Tree couplers are used in cascaded PON architecture
Fiber optic couplers are important optical devices that comprise several common passive fiber
optic devices like optical splitter, optical combiner and optical coupler, as well as active couplers.
These devices are widely used in fiber optic data links whether it is WDM systems or PON (Passive
Optical Network) systems. In a word, fiber optic couplers contribute a lot in applications beyond
point-to-point links [8].
In the chapter 3, the mathematical model of the Y coupler, its characteristics and working will be
discussed theoretically.
14
Chapter 3
Mathematical Model of Optical Y Coupler
Since Y couplers can distribute the input signal evenly or in 60/40, 70/30, 80/20 ration among the
two receivers, depends on their need of power and make possible the interconnection between
them, they become quite useful basic component in high speed, multiple access optical circuits
and networks. They are also quite advantageous constructing large scale optical fiber networks
when its type is free space integrated. In this type, a slab waveguide region is located between the
fan shaped input and output channel waveguide arrays. Here in this chapter its properties are
analyzed by mathematical and theoretical means [9].
3.1 Theory
The Y coupler is a 1x2 fiber optic coupler that has one input and two output ports. Each of them
is being a single mode fiber that supports only the LP01 mode of light at the wavelength of interest.
A 1x2 Y-coupler can be simply modeled with a 2x2 fiber optic coupler having one of the 4 ports
not used such that terminated with an index matching absorber. Figure 3.1 and 3.2 show the
schematic configuration of 2x2 coupler and 1x2 Y coupler respectively. The output array receives
the input power radiated to the slab (free space) region from the channel waveguide. The radiation
pattern at the output side slab–array interface should be uniform over a sector of two waveguides
to get the uniform power distribution into two output waveguides.
Figure 3.1: Coupling light through a 2x2 fiber optic coupler
Figure 3.2: Coupling light through a 1x2 fiber optic coupler
It starts from a simple mathematical model regardless of the light coupling mechanism to describe
the physics of light wave coupling. The time dependence of the light signals at the 3 ports as
En = (½) En exp(-jωt) + c.c.
15
Where, En (n = 1, 2 and 3) are the complex amplitudes of the electric fields and c.c. stands for the
complex conjugate [10].
When two waveguides are brought close together and interact via the evanescent fields outside
their boundaries, cross coupling can occur. This cross coupler is represented by a transfer or unitary
matrix that has a single parameter, c. Maxwell's equations are linear, and therefore the solution of
optical field for one set of excitations can be added to the solution for another set to get the total
field when both excitations are applied. In terms of the amplitudes E1, E2 and E3, it means that
the output amplitudes will be a linear combination of the input amplitude. For an ideal loss-less
1x2 coupler, the output signal is a unitary transformation of the input signal, which can be
conveniently expressed in the form a unitary matrix as follows:
[𝐸2𝐸3
] = 𝑈 [𝐸1]
[𝐸2𝐸3
] = [𝑎 𝑏
−𝑏′ 𝑎′] [𝐸1] 3.1
Where, 𝑎 & 𝑏 are complex values and 𝑎′ & 𝑏′ are their complex conjugate [11].
In the general case, all these values are different & independent, which satisfy the following
normalization constraint
| 𝑎 |2 + | 𝑏 |2 = 1.
However, the Y Coupler component considered here is symmetric and loss-less, and this results in
a reduction of independent parameters to one real variable whose value is between 0 and 1.
This unitary transformation guarantees that the total optical ‘‘power’’ is conserved (loss-less)
|E1|2 = |E2|2 + |E3|2.
From equation 3.1,
[𝐸2𝐸3
] = [𝑎𝐸1 + 𝑏𝐸1
−𝑏′𝐸1 + 𝑎′𝐸1]
[𝐸2𝐸3
] = [𝑎𝐸1 + 𝑏𝐸1
𝑎′𝐸1 − 𝑏′𝐸1] 3.2
When light that goes from E1 to E2 and E3 could be defined as the value of E1=1. Therefore, the
symmetry reduces the four independent coefficients to two. Call the coefficient for the one that
stays in the same waveguide Y, the transfer matrix becomes [10]
[𝐸2𝐸3
] = [𝑎 + 𝑏
𝑎′ − 𝑏′] 3.3
16
3.2 Consequence of conservation of energy
The Y Coupler is loss-less. It is supposed that all the optical power that enters on the port on the
left will make its way to the two ports on the right. The optical power at any of the ports is
proportional to the square of the magnitude of the optical amplitude. Therefore,
E2E2′ + E3E3′ = E1E1′ 3.4
Where, E1′, E2′ and E3′ are complex conjugate of E1, E2 and E3 respectively.
Substituting equation 3.3 into equation 3.4, considering E1 =1 [11],
(𝑎 + 𝑏) (𝑎 + 𝑏) ′ + (𝑎′ – 𝑏′) (𝑎′ – 𝑏′) ′ = 1
(𝑎 + 𝑏) (𝑎′ + 𝑏′) + (𝑎′ – 𝑏′) (𝑎 – 𝑏) = 1
𝑎𝑎′ + 𝑎𝑏′ + 𝑎′ 𝑏 + 𝑏 𝑏′ + 𝑎𝑎′ – 𝑎′ 𝑏 – 𝑎𝑏′ + 𝑏 𝑏′ = 1
𝑎𝑎′ + 𝑏𝑏′ = 1 3.5
3.3 Delay normalization
The time it takes light to cross the coupler results in a delay and it is proportional to the physical
length of the coupler. In the model of an optical circuit, the Y Component is often normalized to
have this delay eliminated. To model a real coupler, it will be necessary to have a delay line in
series with the Y Coupler. The delay line will account for the fact that the plane on which the E1
is defined is in a different place than the plane on which the E2 and E3 are defined. To define the
coefficient, 𝑎, to be a real number and almost independent of λ. ‘𝑎’ is the fraction of light that
stays in the same waveguide after going through the coupler. To normalize it to be a positive real
number, and from conservation of energy, ‘𝑎’ must therefore be between 0 and 1 and it can be
represented instead by another positive real number, c called coupling coefficient, defined as [11]
𝑎 = √1 − 𝑐 3.6
3.4 Y coupler unitary matrix
From equation 3.5, the values of | 𝑎 |2 and | 𝑏 |2 are the coupler’s power splitting ratios for the
parallel and cross paths, usually expressed in percentages.
𝑎𝑎′ + 𝑏𝑏′ = 1
| 𝑎 |2 + | 𝑏 |2 = 1
|√1 − 𝑐||√1 − 𝑐| + | 𝑏 || 𝑏 | = 1
|1-c| + | 𝑏 |2 = 1
| 𝑏 |2 = c 3.7
Since ‘𝑎′’ is taken to be a real number, there are two possibilities for 𝑏:
𝑏 = j√𝑐 or 𝑏 = -j√𝑐
Therefore, the unitary matrix for the Y coupler is
17
𝑈 = [√1 − 𝑐 𝑗𝑝√𝑐
𝑗𝑝√𝑐 √1 − 𝑐] 3.8
Where, p can take the value +1 or -1. It depends on the details of the coupler.
For example, a 50/50 coupler and 60/40 coupler can be described respectively with
c = ½, 𝑈 =1
√2 [
1 1−1 1
] and
c = 0.4, 𝑈 = [ √0.6 √0.4
−√0.4 √0.6]
Also, a 50/50 coupler could be represented by 𝑈 =1
√2 [
1 11 −1
] or 𝑈 =1
√2 [
1 𝑝𝑝 1
]. Depending on
the exact locations where the input and output amplitudes En (n = 1, 2 and 3) are defined.
The matrix U could absorb some additional phase factors. For instance, if E3 for the 3rd port is re-
defined at a different location along the fiber such that E3 has a phase shift of π or E3′ = E3 exp(iπ)
= -E3, equation 3.1, can then be rewritten as
[𝐸2𝐸3
] = [𝐸2
−𝐸3] = [
1 00 −1
] [𝑎 𝑏
−𝑏′ 𝑎′] [𝐸1] = [
𝑎 𝑏𝑏′ −𝑎′
] [𝐸1] 3.9
The transformation matrix has changed after absorbing a phase factor matrix. A global common
phase factor has no significance in the analysis, but relative phases are critical for the interference
outcome. It is very important to keep track of the phase factors in a consistent manner.
For non-monochromatic light waves, the ω dependence of U must be considered. In general, all
matrix elements of U vary with ω, not only the amplitude but also the phase. Variation of
amplitudes | 𝑎 | and | 𝑏 | means the coupler splitting ratios are wavelength dependent. Variation of
the phases, on the other hand, indicates time delay and group velocity dispersion [10].
Chapter 4 will represent the complete design process and the detailed simulation of the optical
50/50 Y coupler and 60/40 Y coupler by OptiBPM software.
18
Chapter 4
Introduction of BPM, OptiBPM, Design Process & Simulation of 50/50 Y
Coupler and 60/40 Y Coupler
This chapter gives the introduction of BPM and OptiBPM software. It describes the design
creation, simulation and its results of 50/50 Y coupler and 60/40 Y Coupler.
4.1 Beam Propagation Method (BPM)
In the investigation of light wave propagation phenomena of linear and nonlinear types in
waveguides which varying axially, the finite difference beam propagating method (BPM) is one
of the most important and strongest techniques. For example, in branching and combining
waveguides, curvilinear directional couplers, tapered waveguides and S-shaped bent waveguides.
It is also quite useful for light pulse propagation analysis of ultra-short light in optical fibers.
Instead of partial derivatives, finite differences are used to solve Maxwell’s equations in the finite
difference BPM which is same for the finite difference time domain method. Therefore, the BPM
is computer intensive in this sense and it is capable of modelling vast range of devices accurately.
While it differs from a full and direct equations’ solutions of finite difference time domain method
in following two ways: In the first, the BPM is done entirely in the frequency domain and only
weak non- linearity can be modelled by it. The second one is that it differs from an envelope
approximation use in the paraxial direction which varies slowly. There are few assumptions in the
BPM such as most of the light or approximately that much light travels (paraxial approximation)
in the direction of an optical axis of the device. As with most of the subject, this axis is considered
as the third space coordinate in OptiBPM software [11].
4.2 OptiBPM
The OptiBPM software system is user friendly and powerful system for design creation of different
integrated and guided optical fiber wave problems on a computer. It is a step by step method of
the light passage simulation through any waveguide medium. An optical field propagation can be
tracked at any point while propagates along a guiding structure in the integrated and fiber optics.
Also, the BPM allows the distribution of light field observed by a computer simulation. The guided
field and the radiation can be examined simultaneously. The OptiBPM also provides easy data
entry to design waveguide layout. The waveguide blocks are contained by this layout environment
known as primitives. The primitives are helpful in easy device design and to configure various
simulations. The graphical layout of project uses a user friendly graphical interface to design
photonic devices. The waveguide primitives, manipulating and editing tools and special layout
regions are included in the design tools provided in menu options and toolbars.
In two dimensional (2D) and three-dimensional waveguide devices, the light propagation is
simulated by the OptiBPM program. These 2D dimensions are vertical or X-direction for
transverse and horizontal or Z-direction for propagation. The 3D dimensions are vertical or X-
direction for transverse, depth in Y-direction and horizontal or Z-direction for propagation [11].
19
4.2.1 Numerical simulations
The BPM is contained as a core element in the OptiBPM processing environment. The mode
solvers compatible with the BPM algorithms are also contained in it. In dielectric media, the light
propagation is governed by the numerical solution of equations and BPM is based on this. To solve
the Helmholtz equation, monochromatic signals are considered by the BPM and based on
Helmholtz equation approximations, the propagation models are used to reduce the processing
time, simplify the simulations and manage computer memory better [11].
4.2.2 2D BPM
The 2D OptiBPM simulator is based on Crack-Nicolson’s finite difference unconditionally stable
method algorithm. Depending on the design, the program options can be customized as follows:
• Simple or full transparent boundary condition (TBC)
• Starting field choice is given as a rectangular field, a waveguide mode, a Gaussian field or a user
field
• At an angle, starting fields can be launched
• A choice is given by the algorithms between TE and TM polarization
• On Padé approximants, Padé (1,1) and Padé (2,2), the wide-angle propagation is based
• Reference refractive index choice as average, modal or user-defined [11].
4.2.3 Scan parameters automatically
To achieve the optimum performance of device, it often needed to repeat the simulations with
different design parameters to find the optimum conditions. Parameter scan calculations which are
loop like and automatic calculations enabled by the OptiBPM. The program names and saves the
result data files sequentially [11].
4.2.4 Mode solvers
The mode solvers in the OptiBPM are 2D and 3D BPM algorithm compatible and employ various
methods as follows:
• For multi-layer planar structures in 2D, Transfer Matrix Method (TMM),
• In 3D, Alternating Direction Implicit (ADI) method
• Finding modes by solving an eigenvalue problem with the Implicitly Restarted Arnoldi Method
The planar structures program is based on multiple boundary conditions solution at dielectric
interfaces between layers [11].
4.2.5 Graphics
To view effective index distribution, field amplitude, phase and other calculated data, the
OptiBPM has state of the art graphics whose graphical features include adding customizable
colors, solid modeling in 3D graphics, color height coding and topographical view of the 3D
graphs. In the waveguide circuit, the signal tracking along selected multiple paths is allowed by a
monitoring window [11].
20
4.2.6 Introduction to optical waveguides
In the photonic devices, the key elements are optical waveguides which perform different
operations on optical signals such as guiding, splitting, coupling, multiplexing, demultiplexing and
switching. By using planar technology such as microelectronics, on one chip different devices like
transmitters, receivers, electronics components, driving electronics and passive waveguides can be
integrated. The performance of waveguide devices relies on various parameters although its
operation is well understood and researched. For example, electrooptic driving conditions, material
data, initial field distribution, wavelength and geometry. These parameters must be optimized
before a device fabrication. To fabricate a chip, the numerous resources are required and because
of this accurate modeling is predominant with the large-scale optoelectronics.
The design of optical waveguide relies on waveguide modes, mode coupling, simulating light
signals propagation and loss and gain. The waveguide device is defined by the one part of the entry
data of material constants, geometry and fabrication parameters. The waveguide data with a layout
of project will be best to enter using software and the fabrication parameters can also be handled
by it. The numerical calculations configuration is another part of entry data. The numerical
calculation details are limited or hided by the entry systems ideally. However, the designer must
know the underlying numeric aspects because the sophisticated numerical algorithms are used by
waveguide modelling. The building blocks of photonic circuits are waveguides and its width
(constant or variable) is defined perpendicular to the path along the waveguide center.
The applications such as profile designer, OptiBPM layout designer, OptiBPM simulator and
OptiBPM analyzer are consisted in OptiBPM software. In the designer, a design project is created
and saved as a .bpd file initially. In the next step, the simulator simulates the project and displays
the data results depending on selected optical field, path monitor, refractive index or cut view from
a graph generated by simulation. Finally, in the analyzer, the results of simulation can be viewed
which also enables to analyze the results of simulation using a set of tools, review the distributions
optical field and refractive index graphically, display the layout at any iteration step and export
data into ASCII formatted files [11].
4.3 Design process of 50/50 Y coupler
The OptiBPM layout designer has the Graphical User Interface (GUI) which consists main layout,
project layout and notification/error window. All the toolbars and menus are contained by the main
layout to create a project. In the main layout, project layout opens and by using this, waveguides
can be added or edited, the input lane can be inserted and the layout grid look and the layout
magnification can be adjusted. The notifications and error messages are displayed in the
notification/error window.
To design Y coupler, the basic parameters are specified in the initial properties dialog box (Figure
4.3.1). Mostly, they are less frequently changed and that is why set at the beginning of the project
creation and can be changed later if needed. For example, in the design session, the refractive index
property is expected to change less frequently than some other engineered properties such as layout
geometry. The waveguide geometry specification in the transverse plane is called the profile. The
fibers have circular cross section, channel waveguides consist of layers and diffused waveguides
21
have graded index. These definitions less changeable than layout geometry in a design. Therefore,
the width of the default waveguide is written as 2.8µm and its initial profile is set as default
ChannelPro1.
Figure 4.3.1: Initial properties dialog box
By using the profiles and materials tool and from the profile designer window, two isotropic
materials- “guide” and “cladding” is created with the refractive indices of 3.3 and 3.27 respectively
under dielectric section. The new “channel” is created under profiles section with the use of new
material “guide”. These can be seen from Figure 4.3.2 to 4.3.2c.
Figure 4.3.2: Profile designer window
Figure 4.3.2a: Dielectric “guide” dialog box
22
Figure 4.3.2b: Cladding definition
Figure 4.3.2c: Defined channel profile
In the initial properties dialog box of layout designer window, profile is changed to new created
“channel” for default waveguide (Figure 4.3.3).
Figure 4.3.3: Initial properties dialog box—Default waveguide tab
In this dialog box, the wafer dimensions are set to specify the size of the region of analysis in the
planar view with the length of 3000µm and width 60µm and wafer refractive index material is set
to “cladding” to specify what material to associate with any point that is not contained inside a
waveguide (for 2D calculations), which can be seen from Figure 4.3.3a and 4.3.3b. After these
settings the wafer layout can be seen in the layout window (Figure 4.3.4).
23
Figure 4.3.3a: Initial properties dialog box—Wafer dimensions tab
Figure 4.3.3b: Initial properties dialog box - 2D wafer properties
Figure 4.3.4: Layout window
The basic structure of Y coupler is created by using two linear waveguides with the following
linear waveguide properties (Figure 4.3.4a to 4.3.4d):
First linear waveguide with starting offset values in µm for horizontal: 0, vertical: 0 and ending
offset values are horizontal: 100 and vertical: 0. Second linear waveguide with starting offset
values in µm for horizontal: 100, vertical: 0 and ending offset values for horizontal: 3000, vertical:
0 and width: 48.
24
The position and the size of the linear waveguide change according to the settings selected and the
basic coupler layout is shown in Figure 4.3.5.
Figure 4.3.4a: Start offset values - First waveguide
Figure 4.3.4b: End offset values - First waveguide
25
Figure 4.3.4c: Start offset values - Second waveguide
Figure 4.3.4d: End offset values - Second waveguide
Figure 4.3.5: Basic coupler layout
26
The input plane is inserted as close as possible to the layout window’s left side and positioned
more accurately by setting offset 2.0 for Z position under global data tab. Also, the input field is
added with amplitude 1.0 and phase 0.0 of the waveguide under input fields 2D tab. These are
shown in Figure 4.3.6a and 4.3.6b. The modified basic structure is shown in Figure 4.3.7.
Figure 4.3.6a: Input plane properties dialog box
Figure 4.3.6b: Item on input fields 2D tab
Figure 4.3.7: Modified basic structure of coupler
The calculate 2D isotropic simulation is run with the following simulation parameters settings
(Figure 4.3.8):
On the global data, the number of displays set to 250. On the 2D tab, polarization is selected as
TE, the mesh – number of points are set to 600, BPM solver is selected as Padé(1,1), the engine is
selected as finite difference, the scheme parameter is set to 0.5, the propagation step is set to 1.55
and the boundary condition is set to TBC.
27
Figure 4.3.8: Simulation parameters dialog box - 2D tab
The type of view for the simulation can be selected while simulation is running, at the bottom of
the simulation window by using following tabs:
•Optical Field (Height Plot or Image Map)
•Refractive Index (Height Plot or Image Map)
•Cut View
Figure 4.3.9: Simulation - 2D optical field
When the simulation is finished, from the OptiBPM Simulator (Figure 4.3.9) and OptiBPM
Analyzer, the results are reviewed and decides the most efficient and reasonable coupling region.
In the design of 50/50 Y coupler, the two local maxima of intensity and their precise horizontal
positions are important (Figure 4.3.9a).
Since there are 250 displays and the total length is 3000μm, it is recognized that these local maxima
laid within the interval of 2800μm from the origin approximately.
28
Figure 4.3.9a: OptiBPM analyzer- Optical field propagation XZ slice
To view the maxima, data clamping is performed in the analyzer and under E axis, scale min set
to 0.4 and scale max set to 0.5 are selected which represents the intensity for the interval <0,1>.
This can be seen in Figure 4.3.9b and the simulation with new data clamping settings is shown in
Figure 4.3.9c.
Figure 4.3.9b: Data clamping dialog box
Figure 4.3.9c: Simulation with new data clamping settings
By region of interest dialog box, these two points can be viewed more accurately with the values
at Z axis, starts from 200 and ends at 250 and at X axis, starts at 0 and ends at 599(Figure 4.3.9d).
29
Figure 4.3.9d: Region of interest dialog box
The length and width of the graph is recalculated in points instead of in μm by the settings typed
for the region of interest. Therefore, as shown in Figure 4.3.9e, the length changed from 3000μm
to 251 points, and the width changed from 60μm to 600 points.
Figure 4.3.9e: Simulation - Region of interest
This view showed that the two points centers are at 2699μm on the Z axis approximately.
In the designer, the length of the second waveguide is changed to 2699µm and wafer length is
changed to 2800µm to focus on the centers of the points more precisely as shown in Figure 4.3.10
and 4.3.10a and modified layout can be seen in Figure 4.3.10b. After running the simulation again
and performing the all above procedures in a similar manner, a vertical position for the upper and
outermost local intensity maxima found approximately 12.85µm.
30
Figure 4.3.10: Modified second linear waveguide properties
Figure 4.3.10a: Modified wafer properties
Figure 4.3.10b: Modified layout
4.3.1 Drawing the output waveguides and viewing the simulation results in OptiBPM
analyzer
The output waveguide is drawn on the top of the second waveguide and the line that connects the
start point of the second waveguide to the intensity maxima. So, horizontal: 100 and vertical: 0
from start offset to horizontal: 2800 and vertical: 12.85 end offset.
31
The path is assigned to the output waveguide by using the path monitor and the defined path turned
into red waveguides in the layout window (Figure 4.3.11 to 4.3.11b).
Figure 4.3.11: Start offset value – First output waveguide
Figure 4.3.11a: End offset value – First output waveguide
32
Figure 4.3.11b: Red waveguides
The global normalization and power overlap integral with fundamental mode are selected for the
simulation in the layout window as shown in Figure 4.3.11c.
Figure 4.3.11c: Path monitoring type
After running the simulation, the output waveguide is examined in the simulator and in the
analyzer. The goal of the maximum coupling from the local intensity maxima to the output
waveguide and the maximum level of the highlighted section of the graph are fulfilled. This is
done by changing the output waveguide vertical end point precisely. From the path monitor
iteration in the analyzer, it is found that when the vertical end point value is 12.85μm, the coupling
is best (Figure 4.3.12 to 4.3.12e).
Figure 4.3.12: Simulation with output waveguide
33
Figure 4.3.12a: Analyzer layout with output waveguide
Figure 4.3.12b: Analyzer – Optical field propagation XZ Slice with output waveguide
Figure 4.3.12c: Analyzer – Path monitor iteration
34
Figure 4.3.12d: Data clamped view with output waveguide
Figure 4.3.12e: Refractive index propagation XZ slice with output waveguide
To complete the 50/50 Y coupler and since the distribution of the maxima is symmetrically, a
simple trigonometric calculation provided the settings of start offset for horizontal: 100 and
vertical: 0 and end offset for horizontal: 2800 and vertical: -12.85 to use for the second output
waveguide (Figure 4.3.13 to 4.3.13a).
35
Figure 4.3.13: Start offset value – Second output waveguide
Figure 4.3.13a: End offset value – Second output waveguide
With these final settings of output waveguides, optical 50/50 or 1x2 coupler is designed which can
be seen in Figure 4.3.14. Also, its simulation results and analyzer results are shown from Figure
4.3.14a to 4.3.14e and 4.3.15 to 4.3.15e.
36
Figure 4.3.14: Final settings of output waveguides – 50/50 optical Y coupler
Figure 4.3.14a: 50/50 Y coupler simulation result
Figure 4.3.14b: 3D view of 50/50 Y coupler
37
Figure 4.3.14c: 2D view of 50/50 Y coupler
Figure 4.3.14d: Path Monitor of 50/50 Y coupler
Figure 4.3.14e: Cut view of 50/50 Y coupler
38
Figure 4.3.15: Analyzer layout of 50/50 Y coupler
Figure 4.3.15a: Analyzer optical field propagation XZ slice of 50/50 Y coupler
Figure 4.3.15b: Analyzer – Path monitor iteration
39
Figure 4.3.15c: Data clamped view
Figure 4.3.15d: 3D view
Figure 4.3.15e: Refractive index propagation XZ slice
4.3.2 Generating data script
The data script can be generated by selecting Generating Scattering Data Script from the simulation
menu or by selecting the Scattering Data Script button. Then, the scattering data script overwrites
the current data script. The scanning interval and in this interval, the number of steps is determined
by selecting the initial and final values of wavelength to 1.53 and 1.55 respectively and steps: 10
in the Input Plane of the Scattering data dialog box. The final scattering data script is displayed in
the scripting window (Figure 4.3.16).
40
Figure 4.3.16: Scripting window- Scattering data script
41
Again, the simulation has performed with the Simulate Using Script and Simulation generates
scattering data information check box enabled. Also, the wavelength box displays
SMatrixWavelength instead of its numerical value selected previously for the wavelength interval
(Figure 4.3.16a).
Figure 4.3.16a: Simulation parameters dialog box
The simulation results can be seen from Figure 4.3.16b to 4.3.16e with 10 iterations.
Figure 4.3.16b: Scripted 50/50 Y coupler simulation result
42
Figure 4.3.16c: Path monitor of scripted 50/50 Y coupler
Figure 4.3.16d: Analyzer optical field propagation XZ slice of scripted 50/50 Y Coupler
43
Figure 4.3.16e: Analyzer – Path monitor iteration
4.3.3 Exporting scattering data
The scattering data script or S-data can be exported by two ways: first, in Cartesian Coordinates
which exports √𝑃 cos(Φ); √𝑃 sin(Φ) for each waveguide and in Polar Coordinates which exports
√𝑃; Φ for each waveguide. These both are mathematically equal, but the export of polar
coordinates is advantageous for relatively longer devices because of the further interpolation of
slowly varying cumulative phase in OptiSystem software. Therefore, only a few iteration steps are
required. In the cases of sudden phase changes in the device behavior, the cartesian coordinates
export is worth using. However, quite number of iterations is needed for this as in most cases,
frequently oscillating function is to be fitted.
The scattering data is exported in Polar Coordinates from the Export menu of OptiBPM Analyzer
and the exported data file saved as an Y_Coupler_Script.s.
4.4 Design process of 60/40 Y coupler
The 60/40 Y coupler has been designed by performing the same procedure used for the 50/50 Y
coupler in section 4.3. The data used for designing 60/40 Y coupler is as follows and can be seen
from Figure 4.4.1 to 4.4.6c.
Wafer Dimensions: Length (µm): 2800 and Width (µm): 80
First Linear Waveguide Properties (µm): Start Offset Values, Horizontal: 0, Vertical: 0, End Offset
Values: Horizontal: 100, Vertical: 0 and Width: 2.8
Second Linear Waveguide Properties: Start Offset Values, Horizontal: 100, Vertical: 0, End Offset
Values: Horizontal: 2150, Vertical: 0 and Width: 48
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First Linear Output Waveguide Properties: Start Offset Values, Horizontal: 100, Vertical: 0, End
Offset Values: Horizontal: 2800, Vertical: 19.75 and Width: 13
Second Linear Output Waveguide Properties: Start Offset Values, Horizontal: 100, Vertical: 0,
End Offset Values: Horizontal: 2800, Vertical: -15 and Width: 26
Y_Coupler_6040_Script.s, scattering data script is generated.
Figure 4.4.1: Initial properties dialog box—Default waveguide tab
Figure 4.4.1a: Initial properties dialog box—Wafer dimensions tab
Figure 4.4.2: Start offset values - First waveguide
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Figure 4.4.2a: End offset values - First waveguide
Figure 4.4.2b: Start offset values - Second waveguide
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Figure 4.4.2c: End offset values - Second waveguide
Figure 4.4.2d: Start offset value – First output waveguide
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Figure 4.4.2e: End offset value – First output waveguide
Figure 4.4.2f: Start offset value – Second output waveguide
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Figure 4.4.2g: End offset value – Second output waveguide
Figure 4.4.3: Final settings of output waveguides – 60/40 optical Y coupler
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Figure 4.4.4: Scripting window- Scattering data script
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Figure 4.4.5: Scripted 60/40 Y coupler simulation result
Figure 4.4.5a: Cut view
Figure 4.4.5b: Path monitor
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Figure 4.4.6: Analyzer layout of 60/40 Y coupler
Figure 4.4.6a: Analyzer optical field propagation XZ slice of 60/40 Y coupler
Figure 4.4.6b: Refractive index propagation XZ slice
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Figure 4.4.6c: Analyzer – Path monitor iteration
The Chapter 5 will give the brief introduction of OptiSystem software and the use cases of both
the 50/50 and 60/40 Y couplers.
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Chapter 5
Use Cases of Optical 50/50 Y Coupler and 60/40 Y Coupler with the
comparison of 1x2 Power Splitter by using OptiSystem
In this chapter, two use cases of optical fiber 50/50 Y coupler and 60/40 Y coupler each have been
discussed by its comparison with 1x2 power splitter.
OptiSystem is an optical communication system simulation package for the design, testing, and
optimization of virtually any type of optical link in the physical layer of a broad spectrum of optical
networks, from analog video broadcasting systems to intercontinental backbones. A system level
simulator based on the realistic modeling of fiber-optic communication systems, OptiSystem
possesses a powerful simulation environment and a truly hierarchical definition of components
and systems. Its capabilities can be easily expanded with the addition of user components and
seamless interfaces to a range of widely used tools. OptiSystem is compatible with Optiwave's
OptiAmplifier and OptiBPM design tools [12].
5.1 Use case 1: 50/50 Y coupler
By using the CW Laser of frequency 193.1 THz and power 0 dBm, the light has been directed to
1x2 power splitter in which its power ratio is set to ‘5 5’ indicates 50/50 equal power division. At
the two ends of this power splitter, two optical power meters have been attached to measure the
power. It should be noted that, in this case, loss is not considered and this system is working in an
ideal condition. These is shown from Figure 5.1.1 to 5.1.1b.
Figure 5.1.1: Simple use case of 1x2 power splitter
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Figure 5.1.1a: CW laser properties
Figure 5.1.1b: 1x2 power splitter properties
After calculating the whole project as shown in Figure 5.1.2, the optical power of 500 x 10-6 W
at the both ends of power splitter can be viewed (Figure 5.1.2a).
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Figure 5.1.2: Calculation output
Figure 5.1.2a: Optical power meters reading
To use the exported Y_coupler_script.s of 50/50 Y coupler in the OptiSystem, OptiBPM
Component NxM is selected from the component library in the OptiSystem file. In the file format
row of the OptiBPM Component NxM properties, value is selected as Amplitude Phase for polar
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coordinates and in the filename row, Y_coupler_script.s file is imported as a value. Finally, the
Label of the component is changed to Y_coupler_5050 and the same system is created as for 1x2
power splitter. These can be seen in Figure 5.1.3 and 5.1.3a.
Figure 5.1.3: OptiBPM NxM component properties for 50/50 Y coupler
Figure 5.1.3a: Simple use case of 50/50 Y coupler
The result of optical power meter after running the program is shown in Figure 5.1.4. The output
coupled at both the terminals of Y coupler is equal, i.e. 354.903 x 10-6 W, after considering the
losses. It can be said that 50/50 Y coupler is working same as a 1x2 power splitter.
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Figure 5.1.4: Optical power meters reading
5.2 Use case 2: 60/40 Y coupler
The same steps are repeated for this case and power ratio array is set to ‘6 4’ means 60/40 for the
power splitter and Y_Coupler_Script.s is used. The power splitter splits the power in 600 x 10-6
W and 400 x 10-6 W ideally and 60/40 Y coupler couples light into 60/40 power ratio of 70.419 x
10-6 Wand 57.149 x 10-6 W after considering the losses (Figure 5.2.1 to 5.2.2b).
Figure 5.2.1: 1x2 power splitter properties
Figure 5.2.1a: Optical power meters reading
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Figure 5.2.2: OptiBPM NxM component properties for 60/40 Y coupler
Figure 5.2.2a: Simple use case of 60/40 Y coupler
Figure 5.2.2b: Optical power meters reading
5.3 Use case 3: 50/50 Y coupler
In this case, by using the spatial CW laser of 633nm frequency, light is launched into a parabolic
index multimode optical fiber of 1m length, 100µm of core radius and 25µm of cladding
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thickness, through a thin lens of 10mm focal length and a spatial connector. The attenuation is
set to 2.61 dB/km. At the end of the optical fiber, 1x2 power splitter is connected to split the
power equally and transfers to both the spatial PIN photodiodes. There are three optical power
meters are connected, one at the end of the fiber and two at the power splitter terminals. Also,
electrical power meter visualizers are connected at the ends of both the photodiodes to measure
the electric power. These details are shown from Figure 5.3.1 to 5.3. It can be noted that 1x2
power splitter is lossless.
Figure 5.3.1: 50/50 power splitter system
Figure 5.3.1a: Spatial CW laser properties
Figure 5.3.1b: Thin lens properties
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Figure 5.3.1c: Spatial connector properties
Figure 5.3.1d: Parabolic index multimode fiber properties- Main
Figure 5.3.1e: Parabolic index multimode fiber properties- Fiber profile
Figure 5.3.1f: 1x2 power splitter properties- ‘5 5’
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Figure 5.3.1g: Spatial PIN photodiode properties
The results of all optical power meters and electrical power meters after calculation of the whole
project, can be seen in the Figure 5.3.2.
Figure 5.3.2: Optical power meters and electrical power meters reading
The same circuit is used to establish a system using 50/50 Y coupler and its results are shown in
Figure 5.3.3 and 5.3.3a respectively.
Figure 5.3.3: 50/50 Y coupler system
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Figure 5.3.3a: Optical power meters and electrical power meters reading
5.4 Use case 4: 60/40 Y coupler
In this case, power splitter ratio is set to ‘6 4’ means the 60/40 power system is set up (Figure
5.4.1) and the readings of optical power meters and electrical power meter visualizers are shown
in Figure 5.4.1a.
Figure 5.4.1: 1x2 Power splitter properties- ‘6 4’
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Figure 5.4.1a: 60/40 power splitter system and its power meters readings
Also, the 60/40 Y coupler has been set up in the similar as above (Figure 5.4.2) and its results are
shown in Figure 5.4.2a.
Figure 5.4.2: 60/40 Y coupler system
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Figure 5.4.2a: Optical power meters and electrical power meters reading
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Conclusion & Summary
The Y couplers are ideal for optical fiber communication systems, fiber to home technology and
antenna networks because of their several benefits such as high reliability and stability, dual
operating window, low excess and polarization dependent losses and high directivity. This project
gives the mathematical model of optical Y coupler with the consequence of conservation of energy
and delay normalization. The introduction of Beam Propagation Method (BPM), OptiBPM and
OptiSystem software are given in detail. The step by step design processes of 50/50 coupler and
60/40 coupler are explained with their complete simulation results. These simulation results are
analyzed in OptiBPM Analyzer and it shows the optical field propagation of both the designed Y
couplers with their different clear views of light propagation path, such as path monitor iteration,
3D view, data clamped view, refractive index view. Then, the designed Y couplers are scripted
and simulated again, to export for the utilization in optical fiber networks. In the OptiSystem
software, two different network circuits have been created by using the designed 50/50 Y coupler
and 60/40 coupler and their results have been compared to the same circuit created by using
OptiSystem component (ideal 1x2 power splitter). From the analysis, it can be seen that 50/50 Y
coupler and 60/40 Y coupler display similar power outputs at the terminals as of 50/50 and 60/40
1x2 power splitters respectively, but note that 1x2 ideal power splitter is lossless, while both the
designed couplers have some losses. Future work need to be continued to reduce such losses.
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References
[1] Fundamentals of Photonics, Module 1.8, Fiber Optic Telecommunication, Nick Massa,
Springfield Technical Community College, Springfield, Massachusetts
[2] https://www.techopedia.com/definition/30643/fiber-optic-coupler
[3] http://www.fiber-optics.info/articles/couplers_splitters
[4] International Journal of Electronics and Communication Engineering, ISSN 0974-2166
Volume 4, Number 5 (2011), pp. 473-482
[5] Optical Fiber Communications, Principles and Practice, third edition, John M. Senior assisted
by M. Yousif Jamro
[6] Fiber-Optic Communication Systems (3rd ed, 2002), Govind E Agrawal
[7] https://www.fiberoptics4sale.com/blogs/archive-posts/95047750-optical-fiber-couplers
[8] Introduction to fiber optics by Ajoy Ghatak & K. Thyagarajan, Cambridge University Press
[9] Fundamentals of Optical Waveguides, Second Edition, Katsunari Okamoto, Okamoto
Laboratory Ltd Ibaraki, Japan
[10] Fiber Optic Sensing and Imaging, Fiber Optic Interferometric Devices, Utkarsh Sharma and
Xing Wei
[11] OtptiBPM Waveguide Optics Design Software Tutorial
[12] OptiSystem Software Tutorial