Chapter 1
Introduction
1.1 Background
At present, optical fiber is one of the most popular media used in the
telecommunication system resulting from its light weight, small size, low
transmission power loss and high bandwidth comparing with conventional copper
media. Thus, optical telecommunication networks are used for providing services
which require high bandwidth such as broadcasting cable television to subscribers,
data links between telephone exchanges or internet gateway links to other countries.
An optical fiber coupler, or coupler in short, is a device that distributes light
passively from a main fiber into another fiber. It can be used as a power
divider/combiner
or
wavelength
division
multiplexer/demultiplexer
in
tele-
communication networks. In a growing market of Fiber to the Home (FTTH) services
using Passive Optical Networks (PON), couplers remain key components in splitting
and combining both downstream and upstream signals [1]. Optical fiber couplers are
also used in remote fiber monitoring system for Passive Optical Network (PON) and
Fiber to the Home (FTTH) system to detect troubles of the installed optical cables [2].
1.2 Optical fiber coupler fabrication method
The Fused Biconical Tapered (FBT) method is most widely used for coupler
fabrication [3]. This method is carried out by fusing two twisted single-mode fibers
together while elongating them to reduce the fiber diameter and the light from one
fiber is coupled to another fiber as shown in Figure 1.1.
2
1st optical fiber
2nd optical fiber
Pull
Pull
Burner
Figure 1.1 Optical fiber coupler fabrication method.
Optical characteristics including insertion loss and coupling ratio can be
computed from the optical power of the transmitted light measured during the
fabrication of the coupler [4].
Insertion loss is the power attenuation level of the signal when a coupler is
connected into the optical network. For example, if a coupler with 3 dB Insertion loss
is connected to a 10 dBm (10 mW) light source, the light power at the output port of
the coupler will become 7 dBm (5 mW) as shown in Figure 1.2.
The light power in dBm unit can be computed by
 Power(W ) 

PowerdBm  10 log 10 
 1mW 
(1.1)
which is the power compared with the power of 1 mW light. Thus, the light
with power level of 0 dBm has the same power level as 1 mW light.
3
10 dBm
7 dBm
(10 mW)
(5 mW)
7 dBm
Insertion loss: 3 dB
(5 mW)
(50%:50% splitting ratio)
Figure 1.2 Insertion loss of coupler.
Coupling ratio is the ratio of light coupled from one fiber to another fiber
inside the optical fiber coupler. For example, if 100% of light is launched into the
input port of coupler with 20%:80% splitting ratio, the light power will remains in the
same fiber at 80% of original power and 20% of original light power will be coupled
to another fiber as shown in Figure 1.3.
80%
100%
Coupler with 20%:80% splitting ratio
20%
Figure 1.3 Coupling ratio of coupler.
Both optical characteristics of coupler changes throughout the fusion and
elongation process of the coupler. The computer which acts as a control unit will
monitor and compute the optical characteristics continuously until certain coupling
ratio and insertion loss are achieved and the fabrication process will be stopped. Then,
the coupler will be packaged in the reinforcement material to protect fused and
elongated area of the fibers.
1.3 Limitation of current measurement system
The optical characteristics are monitored at the wavelengths which are
representative of the optimal wavelength region of each coupler [5]. In this process, as
shown in Figure 1.4, two light sources of different wavelengths are connected to both
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input ends of two optical fibers to be fused together. The output ends of the fibers are
connected to two sensors of an optical power meter. The power readings are sent to
the control program to compute the characteristics.
Light source #1
Sensor-A
Sensor-B
Light source #2
Optical
Power
H2 flame
Meter
Figure 1.4 Conventional two wavelength light measurement system.
In each power reading cycle, one light source will be turned on at a time. The
wavelength of the optical power meter is then updated to match with that of the input
light source and wait for the power of the light source to be stable before each
reading. After the reading is finished, the light source will be turned off and start the
next power reading cycle of next wavelength by repeating the steps mentioned above.
The time required for reading the optical power of 2 wavelengths is 6 seconds per
reading cycle.
With this configuration, one can monitor the optical characteristics at only
2 wavelengths, usually 1310 nm and 1550 nm, while the specification required the
optical characteristics of the coupler as wavelength range. For example,
1290 – 1330 nm range and 1530 – 1650 nm range. Although it is possible to monitor
the optical characteristics at other wavelength, the wavelength of the light source must
be changed and the measurement is taken one wavelength at a time. As a result, the
fabrication process becomes less efficient.
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1.4 Proposed Solutions
To overcome such limitation in the fabrication process, a system that can
monitor optical characteristics of the coupler at multiple wavelengths simultaneously
must be developed. The optical power reading speed improvement can be achieved by
implementing the direct analog signal reading from photodiode. Additionally, a signal
amplifier circuit is also needed to convert the current received from photodiode to
voltage signal.
Since photodiodes only detect over all power, the information regarding
wavelengths is lost. New detection scheme also need to be developed to obtain the
information of wavelength received by photodiode.
The output of light source currently available in the market can be selected as
either continuous light or amplitude-modulated light [6] with internal/external signal
source. In most light sources, the waveform of output light using internal signal
modulation option is 270 Hz square wave. However, the output light from the light
source using external modulation option depends on the shape of the signal given to
the light source.
The proposed system uses 3-wavelength light source which employs 3 light
sources amplitude modulated by 3 different square waves as the input light signal of
the fabrication process as shown in Figure 1.5.
Signal generator f1
1310
nm
Fabrication process #1
WDM
WBC
Signal generator f2
1550
nm
Signal generator f3
1630
nm
Figure 1.5 3-Wavelength light source system.
Fabrication process #2
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For the optical power reading portion, photodiodes are used together with
logarithmic amplifier circuits [7], instead of transimpedance amplifier circuits which
are normally used for amplifying signal from photodiodes, and connected to an
analog-to-digital converter of the control computer. The detected signal is then
analyzed to obtain the optical power of the received light signal using the control
program that is newly developed as shown in Figure 1.6.
3-wavelength
Photodiode
Light source
Photodiode
Logarithmic Amplifier
H2 flame
Analog to digital
converter
Computer
Figure 1.6 The proposed measurement system.
1.5 Literature reviews
TAKAHIRO [8] studied an alternative optical power reading method by
reading the optical power directly from photodiode instead of using conventional
optical power meter. The trans-impedance amplifier circuit is used for converting the
current received from photodiode to voltage. The output voltage from the amplifier is
connected to the analog signal acquisition port of the computer and the optical power
is computed. Optical dynamic range of the prototype system could reach to 40 dB
using automatic gain selection of 2 gain setting circuit. However, the transimpedance
circuit must have more gain setting ability to support wider dynamic range.
NEWMAN [9] proposed a new method to monitor the optical power in the
DWDM optical network system by using logarithmic amplifier circuit instead of
trans-impedance amplifier circuit. The optical power in dBm unit can be computed
directly from the output voltage of the logarithmic amplifier circuit since its transfer
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function is expressed in dBm/V, while the transfer function of the trans-impedance
amplifier is expressed in Watt/V which requires further calculation to get the optical
power in dBm. Logarithmic amplifier circuit can achieve 80 dB optical dynamic
range without any changing on gain setting of the circuit. While trans-impedance
circuit would need at least 3 gain setting circuits to meet the same level of dynamic
range performance.
NYKOLAK et al. [10] studied a three-wavelength add-drop multiplexer
(ADM) node, composed entirely of passive fiber components. This node could
simultaneously add or drop three wavelength channels. The ADM used four 3-dB
fiber couplers, a fiber Bragg reflection filter, and four fiber biconic taper filters. The
wavelengths used in the experiment were 1555.9 nm, 1557.8 nm and 1554.0 nm.
Since the wavelengths used were very close to each other, there were some cross talk
penalties approximately 0.2 dB to 0.4 dB from two other channels. The insertion loss
of each routing channel also varied from 7.4 dB to 9.4 dB.
KUHARA et al. [11] developed a new transceiver module using wavelengthselective coupler and WDM-PD to replace the usage of WDM coupler and a
Y-branch component in conventional module in order to reduce component number
and module size. A new 1.3 µm-WDM-PD had also been developed in order to obtain
high responsivity at 1.3 µm and high transmittance at 1.55 µm. The 1.3 µm WDM-PD
and 1.55 µm-PD was arranged in series along the optical axis so that the 1.3 µm light
will be absorbed by 1.3 µm-WDM-PD and let through the 1.55 µm light to 1.55µmPD. The wavelength-selective coupler was specially fabricated to achieve coupling
ratio of 50%:50% at 1.3 µm and 100% at 1.55 µm. The module assembled with
pigtailed-components was evaluated and the fundamental operation of this module
was confirmed.
MEYER [12] proposed the concept of using a laser diode transmitter and a
photodiode receiver to allow radio signals, as high as 1 GHz in frequency, to be
directly modulated onto optical fiber cable. The amplitude modulated signal could be
transmitted along the optical fiber cable and coupled to a photodiode receiver. The
receiver down converts the optical modulated signal to the original radio frequency
signal which can be amplified to a usable level. A number of parameters were
considered to evaluate the system performance as applied to land mobile radio
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frequency applications. Some of the major parameters were link loss, sensitivity,
dynamic range and linearity. This concept could provide new alternatives to solve
difficult R.F. transmission problem.
ARMSTRONG [13] studied a dual stage fused WDM coupler performance in
the trunk line for 2-channel WDM transmission system in 1310 nm region and
1550 nm region. The advantage of the dual stage WDM over the single stage WDM
was the wavelength isolation. The minimum wavelength isolation of dual stage WDM
was 26.6 dB, about twice of those of single stage WDM which was 13 dB. The field
test had been done in a 50 km installed system operating over a traffic carrying cable
between Burton on Trent and Tamworth, part of the British Telecom Trunk Network.
The system was operated unidirectionally, with 140 Mbit transmission at 1294 nm
and 1548 nm. No receiver power penalty was observed at either receiver.
SENIOR [14] summarized and described advantages/disadvantages of devices
for wavelength multiplexing and demultiplexing. The studies included both passive
and active devices. The passive devices were categorized into prism devices,
diffraction grating devices, integrated waveguide devices, spectral filter devices,
hybrid techniques (combination of diffraction grating and dielectric thin film
interference filter) and fiber coupler devices. The active devices were categorized into
multiple integrated sources and detectors, wavelength tunable lasers, wavelength
tunable filter devices and optical amplifiers.
1.6 Purpose of the study
1.6.1 To improve the performance of the optical characteristic measurement
system at the optical fiber coupler fabrication process by using
3-wavelength light.
1.6.2 To improve the speed efficiency of optical power reading by directly
reading 3-wavelength optical power simultaneously from photodiode.
1.7 Benefits expected
With the usage of existing equipments and minimum investment, the
improved optical characteristic measurement system at optical fiber fabrication
process will be able to inspect the optical characteristic of the couplers during
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fabrication at 3 wavelengths simultaneously in one reading cycle. This will improve
the yield and the capacity of the fabrication machine. Thus, the manufacturing cost of
the optical fiber coupler will be reduced.
1.8 Scope of works
Design and build the optical characteristic measurement system which can
detect and compute the optical power of 3 wavelength lights received by the
photodiode simultaneously during the fabrication of optical fiber coupler. The
accuracy of the developed system should be within the acceptable range for using in
the coupler fabrication.
1.9 Project planning
1.9.1 Design and build a 3-wavelength light source which can combine the
light from 3 light sources with 1310 nm, 1550 nm and 1630 nm
wavelength into one single optical fiber. Each light source will be
amplitude modulated with the signal with pre-defined frequency.
1.9.2 Design and build a logarithmic amplifier circuit to convert the current
received from photodiode to voltage signal with the maximum voltage
value of 5 Volts. The output voltage will be connected to an analog-todigital circuit with 16-bit resolution.
1.9.3 Develop a new method of demultiplexing the optical power information
of the 3 wavelength lights from the voltage signal received.
1.9.4 Develop a new software to compute the optical power detected in dBm
and calibrate the power reading with the existing optical power meter.
1.9.5 Compare the optical characteristic measurement results with those of the
existing system.