Chapter 2
3-Wavelength Light Source Module
2.1 Introduction
This chapter will describe about the components and its underlying techniques
used in developing 3-wavelenght light source module. This module consists of 3 light
sources of different wavelength and optical power setting, 3 waveform generators of
different frequency and 2 optical fiber couplers. Each light source is amplitude
modulated by different waveform generator to obtain unique waveform and all of the
modulated lights are combined together into one optical fiber.
2.2 Light sources
In order to provide the wavelength adjustable function for 3-wavelength light
source module, all the light sources used in this study are tunable laser source. The
wavelength of the output light from each light source can be independently adjustable
within its wavelength range. All the light sources also have the function of amplitude
modulation with external signal generator or internal signal generator to enable newly
developed wavelength detection scheme which will be introduced later in chapter 4
and chapter 5.
2.2.1 Wavelength selection
The wavelengths selected for this study are 1310 nm, 1550 nm and 1630 nm
which cover 3 wavelength regions namely O-Band (1260-1360 nm), C-Band
(1530-1565 nm) and U-Band (1625-1675 nm) [15]. These wavelengths are also
selected based on the optical specification of the highest volume of production
quantity for optical fiber coupler in the factory.
2.2.2 Light source for O-Band
Hewlett Packard 8167B, as shown in Figure 2.1, is a light source for O-Band.
It can provide the wavelength range of 1255 nm to 1365 nm.
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Figure 2.1 Hewlett Packard 8167B.
This light source can be operated with both internal amplitude modulation
mode and external amplitude modulation mode.
For internal amplitude modulation mode, the output light is modulated with
50% duty cycle square wave with the frequency ranging from 250 Hz to 300 kHz and
100% modulation depth (on/off). The amplitude of the output light can be
demonstrated in Figure 2.2.
Figure 2.2 Amplitude of output light from HP8167B operated with
internal modulation mode.
For external amplitude modulation mode, the modulation frequency can be
ranged from 200 kHz to 20 MHz with modulation depth at ±15%. The maximum
amplitude of signal from the signal generator must not exceed 5 Vp-p. The amplitude
of the output light will be changed according to the signal given. For example, the
amplitude of the output light after amplitude modulated with sine wave from external
signal generator is shown in Figure 2.3.
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Figure 2.3 Amplitude of output light from HP8167B after amplitude
modulated with external sine wave generator.
The wavelength of output light from this light source is set at 1310 nm.
2.2.3 Light source for C-Band and U-Band
There are a couple of units which are selected for C-Band and U-Band.
Summary information of each unit is described hereafter.
Multi-channel
tunable
laser
source
for
both
C-Band
and
U-Band
Santec ECL-210 (laser source module)/MLS-8100(main frame) multi-channel
tunable laser, as shown in Figure 2.4, can serves as the light source for both C-Band
and U-Band in one unit. This main frame (MLS-8100) can hosts 8 units of laser
source module (ECL-210) and each laser source module is controlled independently.
In this study, only 2 laser source modules are used. One module with wavelength
range of 1510 nm to 1590 nm is used for C-Band and another module with
wavelength range of 1550 nm to 1630 nm is used for U-Band.
Figure 2.4 Santec ECL-210/MLS-8100.
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This light source can be operated with only external amplitude modulation
mode. The modulation frequency can be ranged from 0 Hz to 10 kHz with modulation
depth at 100%. The maximum amplitude of signal from the signal generator is limited
to 5 Vp-p.
The wavelength of output light from C-Band light source module is set at
1550 nm and 1630 nm for U-Band light source module.
Single-channel laser source for C-Band
Hewlett Packard 8168D, as shown in Figure 2.5, is a light source for C-Band.
It can provide the wavelength range of 1490 nm to 1565 nm.
Figure 2.5 Hewlett Packard 8168D.
This light source can be operated with both internal amplitude modulation
mode and external amplitude modulation mode. The details of both operating modes
are the same as those of Hewlett Packard 8167B described earlier.
The wavelength of output light from this light source is set at 1550 nm.
Single-channel laser source for U-Band
Agilent Technologies 81640A (laser source module)/8164A (main frame), as
shown in Figure 2.6, is a light source that covers both C-Band and U-Band. This main
frame (8164A) can host only one laser source module. This laser source module
(81640A) can provide the wavelength range of 1510 nm to 1640 nm.
14
Figure 2.6 Agilent Technologies 81640A/8164A.
This light source can be operated with both internal and external amplitude
modulation modes.
For internal amplitude modulation mode, the output light is modulated with
50% duty cycle square wave with the frequency ranging from 200 Hz to 300 kHz and
100% modulation depth (on/off).
For external amplitude modulation mode, the modulation frequency can be
ranged from 5 kHz to 20 MHz with modulation depth at ±15%. The maximum
amplitude of signal from the signal generator must not exceed 5 Vp-p. The amplitude
of the output light will be changed according to the signal given.
The wavelength of output light from this light source is set at 1630 nm.
2.2.4 Light sources selection
Four light sources are described in previous subsections. But not all of them
are used at the same time. The light sources are selected and used based on the
modulation operating mode.
Light sources for internal modulation mode
For internal modulation operation mode, 3 single-channel light sources are
selected, one for each wavelength region, as shown in Table 2.1. Each light source is
set with unique modulation frequency. The setting parameters of each light source
will be explained later in chapter 5.
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Table 2.1 Light sources selected for internal modulation mode .
Wavelength region
Light source selected
O-Band (1310 nm)
Hewlett Packard 8167B
C-Band (1550 nm)
Hewlett Packard 8168D
U-Band (1630 nm)
Agilent Technologies 81640A/8164A
Light sources for external modulation mode
For external modulation operation mode, 1 single-channel light source for
O-Band and 1 multi-channel light source for C-Band and U-Band are selected, as
shown in Table 2.2. Each light source is individually amplitude modulated with
unique signal from 3 signal generators, one signal generator for one light source
module.
Table 2.2 Light sources selected for external modulation mode.
Wavelength region
Light source selected
O-Band (1310.nm)
Hewlett Packard 8167B
C-Band (1550.nm)
Santec ECL-210/MLS-8100
U-Band (1630.nm)
Santec ECL-210/MLS-8100
2.3 Signal generators for external modulation mode
The signal generators are needed in order to operate the light source in
external modulation mode. Each light source needs a unique signal waveform. Thus,
3 signal generators are necessary.
NF Electronics Instrument DF-191, as shown in Figure 2.7, is one of the signal
generator selected. This unit can provide the frequency up to 159 kHz with maximum
16
10 Vp-p at load impedance 50 Ω. 4 kinds of waveforms are available; sine, square,
triangle and ramp.
Figure 2.7 NF Electronics Instrument DF-191.
This signal generator is set to generate sinusoidal wave at 200 Hz with
amplitude 5 Vp-p.
Agilent Technologies 33120A, as shown in Figure 2.8, is another signal
generator selected. This unit can provide the frequency up to 15 MHz with maximum
10 Vp-p at load impedance 50 Ω. 4 kinds of waveforms are available; sine, square,
triangle and ramp.
Figure 2.8 Agilent Technologies 33120A.
In this study, two units of Agilent Technologies 33120A are used. They are set
to generate sinusoidal wave at 500 Hz and 700 Hz with the amplitude of 5 Vp-p.
2.4 Optical fiber couplers
In order to combine the light from 3 light sources together, devices called
optical fiber couplers are needed.
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The optical fiber couplers are classified by their functions into 2 main
categories, namely WDM (Wavelength Division Multiplexer) and WBC (Wide Band
Coupler).
2.4.1 WDM (Wavelength Division Multiplexer) type
Main function of WDM type coupler is to multiplex two lights of different
wavelength together into one optical fiber, as shown in Figure 2.9, or to demultiplex
each wavelength from the multiplexed light inside the optical fiber, as shown in
Figure 2.10.
1310 nm
1310 nm
1550 nm
1550 nm
Not used
Figure 2.9 WDM as a wavelength multiplexer.
1310 nm
1550 nm
1310 nm
Not used
1550 nm
Figure 2.10 WDM as a wavelength demultiplexer.
2.4.2 WBC (Wide Band Coupler) type
WBC is used for combining lights from two fibers at the input port and
splitting all the lights from its input port into two fibers of the output port with predetermined coupling ratio, as shown in Figure 2.11.
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1630 nm (50%)
1310 nm (50%)
1550 nm (50%)
1310 nm (100%)
1550 nm (100%)
1630 nm (50%)
1310 nm (50%)
1550 nm (50%)
1630 nm (100%)
Figure 2.11 WBC as a wavelength combiner and splitter.
2.4.3 Couplers used in 3-wavelength light source module
Two couplers are used in this study, one WDM 1310/1550 nm coupler and one
WBC 50%:50% coupler.
WDM 1310/1550 nm coupler is used for combining the light from O-Band
(1310 nm) light source and C-Band (1550 nm) light source together. The typical
insertion loss of this coupler is shown in Figure 2.12. This coupler is selected for this
function due to its low insertion loss at the region of 1310 nm and 1550 nm, typically
less than 0.5 dB.
30
Insertion Loss (dB)
25
20
15
Through Port
10
Cross Port
5
0
1200
1300
1400
1500
1600
1700
Wavelength (nm)
Figure 2.12 Typical insertion loss of WDM 1310/1550nm coupler.
WBC 50%:50% coupler is used for combining the light from U-Band light
source (1630 nm) and the combined light from WDM 1310/1550 nm coupler
(1310 nm and 1550 nm) explained earlier. The typical insertion loss of this coupler is
shown in Figure 2.13. This coupler is selected for this function due to its wide
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wavelength range from O-Band to U-Band. Thus, it can be used for combining
multiple wavelengths together. The typical insertion loss throughout the wavelength
range is less than 3.7 dB
4
Insertion Loss (dB)
3.5
3
2.5
2
Through Port
1.5
Cross Port
1
0.5
0
1200
1300
1400
1500
1600
1700
Wavelength (nm)
Figure 2.13 Typical insertion loss of WBC 50%:50% coupler.
2.5 Connection diagram of 3-wavelength light source module
To build up the 3-wavelength light source module, all the components
explained earlier are connected together as shown in Figure 2.14. Each light source is
connected with its own signal generator to be able to operate in external modulation
mode. The output fiber of O-Band light source and the output fiber of C-Band are
connected to the input ports of WDM 1310/1550 nm coupler. This coupler will
combine the lights from 2 light sources into one output fiber, carrying both 1310 nm
and 1550 nm lights, and connected as one of the input port of WBC 50%:50%
coupler. The output fiber of U-Band light source, carrying 1630 nm light, is connected
to another input port of the WBC 50%:50%. The lights from both input ports are
combined into one output fiber, carrying 1310 nm, 1550 nm and 1630 nm, ready to be
used as 3-wavelength light source for the coupler fabrication system.
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Signal generator
O-Band light source
200Hz
1310nm
Signal generator
C-Band light source
500Hz
1550nm
1310 nm
WDM
1310 nm
1310/1550nm
& 1550
nm
1550 nm
WBC
50%:50%
Signal generator
U-Band light source
700Hz
1630nm
1630 nm
For external
1310 nm
modulation
& 1550 nm
mode only
& 1630 nm
3-wavelength light for
coupler fabrication
system
Figure 2.14 Connection diagram of 3-wavelength light source module.
The total insertion loss of the system after combined with 2 couplers is
approximately 3.5 – 4 dB throughout the wavelength range from 1260 nm to
1630 nm.