Fiber-Grating Add-Drop Reconfigurable Multiplexer with
Multi-channel Using in Bi-directional Optical Network
Chen-Mu Tsaia and *Yu-Lung Lob
a
Dept. of Computer and Communication, Kun Shan University, Tainan, Taiwan.
b
Dept. of Mechanical Engineering, National Cheng Kung University, Tainan, Taiwan.
Submitted to Fiber Optic Technology, July 2006
Corresponding Author’s e-mail: [email protected]
Abstract – A bi-directional reconfigurable scheme of multi-channel-selective optical add-drop
multiplexer (OADM) is proposed in this paper. The bi-directional OADM (BOADM) with
enhancing network capacity and flexibility is better than the conventional unidirectional
OADM. The magnitudes of homodyne and heterodyne crosstalks are analyzed for evaluating
the performance of the proposed BOADM. Also, the power equalization of each signal in
add-drop and output ports is managed properly by implementing filters. As a result, the
BOADM will be found wide applications in dense wavelength-division multiplexing (DWDM)
optical networks and in multi-wavelength cross-connects (WXCs).
Keywords: Fiber Bragg Grating (FBGs), bi-directional Optical Add-Drop Multiplexer
(BOADM)
* Corresponding Author: [email protected]
1. Introduction
Dense wavelength-division multiplexing (DWDM) is one technology to provide
cost-effective and attractive for ring networks. In such networks, the optical add-drop
multiplexers (OADMs) are key components that can provide the basic functions. They allow
1
the optical network to be local transmission/extraction on a wavelength-by-wavelength basis to
optimize the traffic, efficient network utilization, network growth, and to enhance the network
flexibility.
Several types of OADM components have been demonstrated, including fiber Bragg
gratings (FBGs) with circulators [1], Mach-Zehnder interferometers [2], and arrayed waveguide
gratings (AWGs) [3-5]. By using AWG filters to build OADM, it possesses shortcomings in
terms of polarization-dependent wavelength, polarization-dependent loss, relatively low
channel isolation, channel crosstalk to all other channels, and a low figure of merit in noise
performance. On the other hand, Mach-Zehnder interferometer has insertion loss from input
port to output port. It is hence a serious drawback on the insertion loss for the cascaded
Mach-Zehnder interferometers. Among the OADM devices, the FBGs with optical circulators
(OCs) are relatively compact and practically realizable.
In port
drop port
add port
1  3
 2 4
4  2
 3 1
out port
Figure 1 Wavelength-selective add-drop node using tunable fiber gratings [6].
Okayama et al. [6] proposed a tunable add-drop multiplexer that uses two 4-ports OCs
and a pair of multiple identical gratings for each add-drop channel using in the unidirectional
network. They also had demonstrated their proposal applying the fiber grating with a
reflectivity of 95% and a heat controller for tuning a small wavelength range. However, some
essential noises such as the homodyne and heterodyne crosstalks were not discussed in that
paper. For example, the fiber grating with a reflectivity of 95% is not acceptable in the
International Telecommunication Union (ITU) standard. We therefore apply the Okayama et al.
[6] architecture to build a bi-directional reconfigurable optical add-drop node and adopt the
2
practical components to investigate the performance in the proposed bi-directional OADM
(BOADM).
Recently, Kim and Lee [7] utilized several FBGs and a pair of 6-port OCs to build a
BOADM. A pair of 6-port OCs was applied to achieve OADM of three wavelengths in two
directions. The variable attenuators were implemented into the system for the power
equalization of each wavelength signal. As the system increases wavelength signals for
add-drop, the BOADM system increases not only the FBGs but also the port number of OCs.
This is the drawback of such architecture on extending the number of Bragg wavelength
signals.
In this paper, we propose a BOADM scheme that is constituted with the tunable fiber
Bragg gratings (FBGs) and multi-port optical circulators (MOCs). The proposed BOADM can
add-drop either single or multiple wavelength channels dynamically at each optical node in
accordance with the network management. Also, the proposed BOADM has advantages on
extending the number of wavelength signal and managing the power equalization of each signal.
Section 2 describes fundamental OADM principle and function using in ring network. The
architecture of the proposed BOADM is illustrated in Section 3. In Section 4, the performance
of the proposed BOADM is analyzed and evaluated. Finally, some technical remarks of the
paper are concluded in Section 5.
2. Optical Add-Drop Multiplexer Principle and Function
An OADM is four ports optical device which includes input port, output port, drop port,
and add port. Most DWDM systems apply it on optical ring network to add-drop the desired
wavelength in local area. The basic schema of OADM is shown in Figure 2. When network
optical signals (with multiple wavelengths, i.e. 1, 2, …, N) transmit into an OADM device
from input port, the OADM can extract the desired optical signal (i.e. wavelength i) from drop
port. The rest signals will go to the output port to continuously travel to next station.
3
Input
 1,  2, .... ,  N
OADM
Output
 1,  2, .... ,  N
Drop  i
Add  i
Figure 2 Schema of OADM.
The transmitted signals later will not exist any the dropped signal i in following network,
at present, with identically the dropped wavelength i can be put into the network to transmit
local signal from add port. This process first is to modulate the local information into the
wavelength i. By the OADM, the modulated signal can be added from the add port to output
port, to carry local information into optical network. Therefore, the OADM can reduce the
number of wavelengths because it supports the wavelength-reuse function in optical network.
Optical ring network is to use an optical fiber channel to link all transmitter/receiver
nodes so that the system cost is reduced due to only one optical fiber channel used. Let N nodes
be supported in optical ring network. If the system requires that every node can directly
transmit its respectively signal to all other nodes, the system is necessary the number of (N-1)2
wavelengths to support in network. To promote spectrum effectively, one can apply the OADM
to reduce the number of wavelengths. Since the OADM supports the wavelength-reuse function,
most optical ring networks employ it to enhance the transmitted capacity.
When using the OADM in optical ring network, the system only requires N(N-1)/2
wavelengths in network. Figure 3 shows a scheme of optical ring network that is constructed by
4 nodes. Each node employs the OADM to add-drop the desired signals. The optical signal is
along counterclockwise to transmit its information. In such ring network, there are 6 pairs
nodes which include node pairs (1, 2), (1, 3), (1, 4), (2, 3), (2, 4), and (3, 4) and can transmit
and receive signals mutually. Each pair is assigned a wavelength to transmit and receive signals.
Table 1 shows that the wavelengths are used in 4 nodes respectively to add-drop the transmitted
4
signals.
1
2
: Node
4
3
Figure 3 With 4 nodes optical ring network.
Table 1 Disposed wavelengths of 4 nodes in optical ring network.
node
1
2
3
4
transmitted/
received node
2
3
4
3
4
1
4
1
2
1
2
3
add/drop
wavelength
1
2
3
4
5
1
6
2
4
3
5
6
The node pair (1, 2) is to use wavelength 1 to transmit/receive signals. The node pair (1,
3) is to use wavelength 2 to transmit/receive signals. Other node pairs are similarly assigned
wavelength to transmit/receive in the network. The node 1 is to employ wavelength 1, 2 and
3 to transmit the signals to node 2, node 3 and node 4. When the node 2 applies the OADM to
extract the wavelength 1 signal, it can recognize this signal that comes from the node 1.
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Similarly to node 3 and node 4 drops the wavelength signals 2 and 3, respectively.
The node 2 drops the wavelength 1 signal from the optical fiber channel, therefore, there
is no any wavelength 1 signal at the rest route from node 2 to node 1. The node 2 therefore can
modulate its information with wavelength 1 to transmit the node 1 through the OADM that
adds the wavelength 1 signal into optical fiber channel. When the wavelength 1 signal from
the node 2 arrives to the node 1, the node 1 also employs the OADM to extract the wavelength
1 signal and it can acknowledge this signal that comes from the node 2. In such network, there
are 6 same procedures in 4 nodes ring network. Therefore, the system is necessary 6 different
wavelengths to support 6 node pairs. Each node pair bases on its assigned wavelength to
transmit/receive their information.
3. Bi-directional OADM Based on Tunable Fiber Gratings
2 OC
1 6
4
2x2 OSW
1 3 add 5
5 
1
EDFA
VA
drop 5
add 3
drop 3
add 1
drop 1
drop 2
add 2
drop 4
add 4
drop 6
add 6 4 2
Light absortber
EDFA
2' 4' 6'
3
1x4 Coupler
1 3 5
Light absortber
1x4 Coupler
 1  3 5
VA
6 1
1' 3' 5'
5 OC 2
2
  
4 3 2 4 6
6  4 2
Figure 4 The BOADM that can switch three optical channels in each direction [7].
The convention OADM only processes unidirectional network signals. However, another
direction optical fiber channel also can be used to carry optical information to transmit local
6
signal to receiver. If bi-directional optical fiber channel is applied, the total transmitted
wavelengths will be reduced to N(N-1)/4 when N nodes be supported in network. Recently,
Kim and Lee [7] utilized several FBGs and a pair of 6-port OCs to build a BOADM, as shown
in Figure 4. A pair of 6-port OCs was applied to achieve OADM of three wavelengths in two
directions.
In Figure 4, the first process is demultiplexing. When one requirement is to extend a
wavelength channel in the network, this architecture has to expand one port of OCs to satisfy
all wavelengths demultiplexed. In other words, the four-wavelength-selective OADM needs to
employ 7-ports OCs. Therefore, as the system operates more wavelength signals, the BOADM
system shown in Figure 4 not only increases the FBGs but also the ports of OCs. This is the
drawback of such architecture on extending the number of Bragg wavelength signals. In Figure
4 the power loss problem should be managed properly so that the variable attenuators are
implemented into the system for the power equalization of each wavelength signal.
2,4,6,8
 1  5 3 7
1,3,5,7
 7  3 5 1
drop 1,3,5,7
add 2,4,6,8
 2  6 4 8
 8  4 6 2
add 1,3,5,7
drop 2,4,6,8
1,3,5,7
2,4,6,8
Figure 5 Two uni-directional tunable OADMs compose a bi-directional OADM.
According to Okayama et al. [6], a bi-directional reconfigurable OADM can be easily
achieved. As illustrated in Figure 5, it is composed of two 3-ports OCs for connecting two
uni-directional OADMs together as a bi-directional reconfigurable OADM configuration. In
7
this structure, there are two 3-ports OCs, four 4-ports OCs and two pairs of multiple identical
FBGs with eight piezo-electric (PZT) actuators to function 8 channel signals in addingdropping reconfigurablly.
Owing to the complexity in Figure 5, a modified bi-directional reconfigurable OADM is
proposed for reducing the number of OCs. Figure 6 depicts a schematic diagram of the
proposed BOADM. In this architecture, we employ two 7-port OCs and two pairs of multiple
identical FBGs with eight piezo-electric (PZT) actuators. The FBG filters are applied the axial
strain by the PZT actuators to tune the central wavelength so that the wavelength signals can
transmit through the output port of an OC.
2 4 6  8
2  6 4  8
Add 1 3 5 7
1 3 5  7
3 4
5  
2 #1
1
5
6
Add 2 4 6 8 1
7
 7 3
 3 7 2
3
 5 1
Drop 1 3 5 7
1 7
Drop 2 4 6 8
#2 6
4 5
2  4  6 8
1  3  5 7
 8 4 6  2
Figure 6 The schematic diagram of a new bi-directional reconfigurable OADM.
To distinct bi-directional wavelength management of the proposed BOADM in Figure 6,
we utilize the odd and even numbers of the wavelengths to explain the different direction. The
first multiple FBGs involve the fixed wavelengths 1 and 5and the tunable wavelengths 3
and 7 based on PZT actuators. Similarly, The second multiple FBGs involve the fixed
wavelengths 3 and 7 and the tunable wavelengths 1 and 5 based on PZT actuators. When
the odd number wavelength signals: 1, 3, 5 and 7 enter the port 4 of the OC #1, the signals
8
will transmit through the first cascaded multiple FBGs by the port 5 of the OC #1.
The wavelength signals: 1, 35 and 7, thereforeare reflected back into the port 6 of
an OC #1 if the PZT actuators are not tuned to change the Bragg wavelengths in 3 and 7. If
the PZT actuators also do not tune the Bragg wavelengths in 1and 5, the wavelength signals:
1, 35 and 7 are all dropped into the port 7 of an OC #1 for dropping. In the case of the
adding function, it is obvious that the wavelength signals: 1, 35 and 7are added into the
port 1 of an OC #2, and then eventually all go into the port 4 of an OC #2 for output. It is
concluded, for example, if only tuning the PZT actuator in the Bragg wavelength 3, the
wavelength signal 3 will not be dropped into a drop port, and eventually the wavelength signal
3 will go into the port 4 of an OC #2 for output. Therefore, one direction of a reconfigurable
BOADM, as shown in Figure 6, has been achieved. Similarly, the even number of the
wavelength signals: 2, 46 and 8 from the other direction of a BOADM also can be
managed by dynamically tuning PZT actuators in the other pair of multiple identical Bragg
wavelengths 2, 46 and 8 for adding and dropping. As a result, a reconfigurable BOADM
by tuning the PZT actuators, therefore, can be achieved easily.
In the proposal, as the system demands more wavelength channels, we just put more
FBGs between the two 7-ports OCs, and it is unlike the case proposed by [7] that requires a
complicated modification. For example, if the system manages N wavelength signals in the
network, there are 2N FBGs being installed between the two 7-ports OCs for the new BOADM.
It finds that the new proposed BOADM has many advantages on extending the number of
wavelength signals. In addition, each wavelength signal passes the OC #1 and #2 three times
respectively so that the power loss caused by OCs for all signals is the same. Therefore, the
problem of the flatten power in the output and drop ports due to insertion loss from FBG is
only discussed; and also the homodyne and heterodyne crosstalks will be introduced in the
following.
9
4. Performance Evaluation
Owing to the characteristics of transmission and reflection loss of FBG, the output power
variation of the BOADM associates with its various add-drop wavelengths. Thus, the flatten
power is the major consideration in the output and drop ports. As discussed in above, when one
adding-dropping various signals, the insertion losses caused by the optical circulators are all
the same. Therefore, only the insertion losses caused by FBGs are discussed in the following.
Table 2 The passing time of insertion loss from the input to drop port.
Wavelength 1, 2
1R
3T
1R
Times
3T
Total Times
3, 4
2T
1R
2T
1T
1R
1T
8R 8R
5, 6
1T
1R
1T
2T
1R
2T
8R
7, 8
3T
1R
3T
1R
8R
Table 3 The passing time of insertion loss from the add to output port.
Wavelength 1, 2
3T
1R
3T
Times
1R
Total Times
3, 4
1T
1R
1T
2T
1R
2T
8R 8R
5, 6
2T
1R
2T
1T
1R
1T
8R
7, 8
1R
3T
1R
3T
8R
From the input to drop ports as illustrated in Figure 6, the signals transmit through FBGs
six times and reflect twice, as listed in Table 2. Similarly the round trip trace from add to
output port, Table 3 shows that the adding signals experience six times transmission loss and
two times reflection loss. Assuming the insert loss in reflection and transmission of FBG to be
the same, totally, eight times insertion loss are listed in Table 2 and 3, respectively.
Consequently, the dropping signals and outputting signals are obtained the flatten power at the
10
drop and output port. However, it should be noticed that the inputting signals encounter the
difference power decay to the output port.
Table 4 The passing time of insertion loss from the input to output port.
Wavelength 1, 2 3, 4
1R 4T
4T 2T
1R
Times
2T
5, 6
1T
1R
1T
4T
7, 8
4T
3T
1R
3T
Total Times
7R
11R
5R
9R
If the inputting signals are not dropped at the drop port, the different wavelengths will
travel different numbers of FBGs to output port. For example, the inputting signal 1 travels
into the output port by way of one time reflection loss and four times transmission loss, as
listed in Table 4. Also, the signal 7 has one time reflection loss and ten times transmission loss
from input to output port. As a result, the variation of the total insertion loss of the signals
would be arisen at the output port. It is obvious that the more Bragg wavelengths result in the
worst to the power equalization at the output port.
Table 5 Wavelength insertion loss required in
filter 1, filter1’, filter 2, filter 2’, filter 3 and filter 3’.
Wavelength loss
1, 2 3 ,4 5,6 7,8
Filter1, Filter1' 3R 3R 3R 3R 3R 3R 3R 3R
Filter2, Filter2' 6R 6R 2R 2R 4R 4R 0R 0R
Filter3, Filter3' 0R 0R 4R 4R 2R 2R 6R 6R
For equalizing the power of each wavelength at the output port, three pairs of specific
filters at the input, add and drop ports are built as illustrated in Figure 7. According to Table 2,
the maximum loss is eleven times insertion loss in FBGs. The solution of equalizing power is
11
to make channel decay 11R insertion loss in output port. Therefore, the filter1 is designed as
three times insertion loss in FBGs to sum together the eight times insertion loss in Table 3 and
1 3  5 7
Output
Filter1'
Add 2 4 6 8
3 4
5  
2 #1
1
5
1
6
7
7  3
1 7
Filter3'
2  6  4 8
Add 1 3 5 7
Filter3
Filter2
Input
2 4  6 8
Filter1
just placed at the add port to adjust the total insertion loss as eleven times for all signals.
Drop 2 4 6 8
3 7 2
#2 6
3
4 5
5 1
 2 4  6  8
Drop 1 3 5 7
Output
 1 3  5  7
Filter2'
 8 4  6  2
Input
Figure 7 The schematic diagram of a new modified bi-directional reconfigurable OADM
with three pairs of specific filters.
The requirement of the filter2 at the input port is to decay totally 11R power in
wavelength 1, 3 and 5, as shown in Table 5. For example, the signal 1 needs to be increased
six times insertion loss and the signal 5 needs to be increased four times insertion loss, so on
the signal 3 and 7. However, the dropping power dependents on the inputting signal power. If
the filter2 is placed in the input port, the wavelength power will unbalance in the drop port.
The filter3 response therefore is necessary to conform the filter2 inverse in spectral domain, as
illustrated in Table 5. Consequently, all drop channels have 14R insertion loss from the input
signals, and the output signals would be decayed 11R power for all channels from the
inputting-adding signals. If all channels power are equalized in the input and add port, the all
channels power will be the same in drop and output port, respectively. Therefore, the add-drop
wavelengths at the output and drop port could be achieved for the power equalization.
12
Outbad power over Inband power
(dB)
50
40
30
20
Isolation 25dB
Isolation 20dB
Isolation 15dB
10
0
0
10
20
30
40
Number of channels
50
Figure 8 Heterodyne crosstalk versus channel numbers in the drop port.
The magnitudes of homodyne and heterodyne crosstalks will be estimated for evaluating
the performance of the proposed BOADM. The FBGs are employed as filters whose qualities
could affect the homodyne and heterodyne crosstalks. Some important parameters in the FBGs
such as the 3-dB bandwidth, the transmission loss, the isolation, and the reflectivity, therefore,
could be used to analyze the performance of a BOADM. The isolation is the reflectivity of
FBG for out-band. This parameter will determine the power of heterodyne crosstalk from the in
port to drop port. Figure 8 shows the calculated heterodyne crosstalk as a function of the
number of channels with isolation 15, 20, 25 dB of FBG. For ITU standard, the power of
homodyne and heterodyne crosstalk need to limit within 25 dB. In general, the isolation of
FBG is achieving larger 20 dB and the proposed BOADM can obtain the selected at least 30
optical channels for ITU standard.
The other parameter that affects the BOADM performance is the FBG reflectivity. Since
the reflectivity of the FBG is not perfect, the signals on the add port have some leakages into
the drop port. The leakage powers are called the homodyne crosstalk and will influence the
13
decision of transmitted information. When the signals from the add port into the first grating
sequence, some wavelengths will be reflected into the output port by FBGs. However, these
wavelength signals also have some leakage power that transmits through the first grating
sequence and then enters into the second grating sequence. Finally they will be reflected back
into the drop port. As to the homodyne crosstalk, this structure all depends on the high FBG
reflection to reduce the crosstalk. Figure 9 shows the homodyne crosstalk of the signals from
the add to drop port regarding to various reflectivities from 98% to 100% in FBGs. As a result,
for the ITU standard of –30 dB in crosstalks, the FBGs with the isolation 25 dB and the
reflectivity 99.9 % are chosen for implementing in the proposed BOADM system.
loss Power (dB)
31
29
27
25
23
21
19
17
15
98
98.5
99
99.5
100
Reflectivity of FBG (%)
Figure 9 Homodyne crosstalk versus various FBG reflectivities.
5. Conclusions
Since the transmitted capacity is rapidly increasing in the communication networks, the
bi-directional components need to be developed. A novel reconfigurable BOADM that
provides a larger transmitted capacity and lower cost is proposed. Also, it is found that the
proposed BOADM has many advantages on extending the number of wavelength signal as
14
compared to the existing ones. In order to manage the power equalization of each signal at the
drop and output port, the specific filters are designed and implemented into the proposed
BOADM. For the ITU standard of –30 dB in crosstalks, the FBGs with the isolation 25 dB and
the reflectivity 99.9 % are chosen for implementing in the BOADM system. It is conclude that
the proposed BOADM could find important applications in DWDM networks communication
system.
References:
[1]. Bilodeau, F., D.C. Johnson, S. Theriault, B. Malo, J. Albert, and K.O. Hill, “An all-fiber
dense wavelength-division multiplexer/demultiplexer using photo-imprinted Bragg
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[2]. Mizuochi, T., T. Kitayama, K. Shimizu, and K. Ito, “Interferometric crosstalk-free optical
add/drop multiplexer using Mach-Zehnder-based fiber gratings”, J. Lightwave Technol., vol.
16, no. 2, pp. 265-276, February 1998.
[3]. Ishida, O., H. Takahashi, S. Suzuki, and Y. Inone, “Multichannel frequency-selective
switch employing an arrayed-waveguide grating multiplexer with fold-back optical paths”,
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723-724, April 1995.
[5]. Ttachikawa, Y., Inoue, Y., Ishii, M. and Nozawa, T., “Arrayed-waveguide grating
multiplexer with loop-back optical paths and its applications”, J. Lightwave Technol., vol.
14, no. 6, pp. 977-984, June 1996.
[6]. Okayama, H., Y. Ozeki, and T. Kunii, “Dynamic wavelength selective add/drop node
comprising tunable gratings,” Electronics Letters, vol. 33, no. 10, pp. 881–882, May 1997.
15
[7]. Kim, J. and B. Lee, “Bi-directional wavelength add-drop multiplexer using multiport
optical circulators and fiber Bragg gratings,” Photon. Technol. Lettr., vol. 12, no 5, pp.
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Table Listing:
Table 1 Disposed wavelengths of 4 nodes in optical ring network.
Table 2 The passing times of insertion loss from the input to drop port.
Table 3 The passing times of insertion loss from the add to output port.
Table 4 The passing times of insertion loss from the input to output port.
Table 5 Wavelength power loss required in filter 1, filter1’, filter 2, filter 2’, filter 3 and filter
3’.
Figure Listing:
Figure 1 Wavelength-selective add/drop node using tunable fiber gratings [6].
Figure 2 Schema of OADM.
Figure 3 With 4 nodes optical ring network.
Figure 4 The BOADM that can switch three optical channels in each direction [7].
Figure 5 Two uni-directional tunable OADMs compose a bi-directional OADM.
Figure 6 The schematic diagram of a new bi-directional reconfigurable OADM.
Figure 7 The schematic diagram of a new modified bi-directional reconfigurable OADM
with three pairs of specific filters.
Figure 8 Heterodyne crosstalk versus channel numbers in the drop port.
Figure 9 Homodyne crosstalk versus various FBG reflectivities.
Brief Biography:
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Tsai, Chen-Mu was born in Tainan, Taiwan, on October 1974. He received his B.S. degree
from the Department of Electronic Engineering at the National Taiwan University of Science
and Technology in 1999. He received the M.S. and Ph.D. degrees from the Department of
Electrical Engineering at the National Cheng Kung University (NCKU), Taiwan, in 2001 and
2006, respectively. Now, he is an assistant professor at the Department of Computer and
Communication in the Kun Shan University, Taiwan. His research interests are mainly in the
areas of optical communications and optical system design.
Lo, Yu-Lung received the B.S. degree from the National Cheng Kung University, Taiwan in
1985, and the M.S. and Ph.D. degree from Smart Materials and Structures Research Center
(SMSRC) at the University of Maryland, College Park, in 1992 and 1995 respectively, all in
mechanical engineering. After his graduation, he jointed Industrial Technology Research
Institute (ITRI) at Opto-Electronics & Systems Laboratories working on fiber optic smart
structures. He joins a member of the faculty of the Mechanical Engineering Department at
the National Cheng Kung University since 1996. Currently, he is full professor. His research
interests are in the areas of optical communication component design, fiber-optic sensors,
optical techniques in precision measurements, and MEMS. He has authored over 50 technical
publications and filed several patents. Dr. Lo is a member of SPIE and SEM.
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