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Journal of China University of Science and Technology Vol.50-2012.01
The Research of Near-Infrared Electro-Optics
Modulation Coding Technology
近紅外線光電調制技術的研究
陳德請
1
李世文
2
1
卓葆軒
3
2
Der-Chin Chen , Shih-Wen Lee , Zhuo-Bao Xuan3
1
2
中華科技大學電機工程系助理教授
3
1,3
逢甲大學電機工程系副教授
逢甲大學電機工程系大四學生
Department of Electrical Engineering, Feng Chia University
2
Department of Electrical Engineering, China University of Science and Technology
摘 要
近紅外光空間調制系統係由(1)近紅外光空間調制、(2)觸發時間變化的光空間
調制器、及(3)近紅外光源(LED)脈衝調變……等三項技術所結合的三度空間調制
技術，係一種進行近紅外光資料調變與解調及加/解密的創新方法。利用光藕合元
件將脈衝調變光藕合至光纖和光空間調制器，此信號被解調端的光檢知器偵測由
後端電路進行解調。因為使用可以變化三種孔徑大小的光圈及三種工作週期的光
波信號，這個系統根據這樣組合共有 27 種編碼方式。經實驗結果本系統的通訊誤
差在 0.2%以內。本系統優點為因為使用光源具單色光特性不比必使用濾光片，使
用光學編碼所以具保密性與安全性，又不容易被電磁干擾。
關鍵字︰誤碼率 光空間調制器 近紅外光
ABSTRACT
In this paper we demonstrate a new technology in which we use the optical
modulation, trigger duration modulation of the optical modulator, and light source (LED
、LD) pulse modulation to build the near-infrared electro-optics modulation coding
system. Using the optical coupler to let the pulse modulation near infrared (NIR) light
enter into an optical fiber and optical modulator device, the electro-optics modulation
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The Research of Near-Infrared Electro-Optics Modulation Coding Technology
coding signal is then detected by the photodiode. Because of using three changeable
bore sizes of aperture module and three duty cycles’ light wave signals. According to
the arrangement, there are 27 kinds of coding modes in this system. The communication
error rate of the modulation coding system is less than 0.2%. There are some advantages
in the system: (1) Using pulse modulation LED light source, there are no optical filters
required necessary. (2) Because of optical coding, data transmission has high security
and low error.
Key word: error rate, optical modulator, near infrared
I. Introduction
An optical modulator is a device which is used to modulate a beam of light. The
beam may be carried over free space, or propagated through an optical fiber. Depending
on the parameter of a light beam which is manipulated, modulators may be categorized
into amplitude modulators, phase modulators, wavelength modulators, polarization
modulators etc. Often the easiest way to obtain modulation of intensity of a light beam
is to modulate the current driving the light source, e.g. a laser diode. This sort of
modulation is called direct modulation, as opposed to the external modulation
performed by a light modulator for example electro-optic modulator (E-O)[1-2],
acousto-optic
modulator(A-O)[3-4],magneto-optic
modulators
(M-O)[5-6]and
mechano-optical modulators. For this reason light modulators are, e.g. in fiber optic
communications, called external light modulators. Here, we use mechanic-optical
modulators, in which a mechanical element such as a three stage small aperture perturbs
the optical light.
According to “the national defense and commercial information security”, the
problems we faced are that the targets of terrorism range from national security to the
privacy of personal information. Terrorists might attack computer system of traffic
control and other public facilities, so we should do our best to counter these potential
threats which include virus prevention, firewall of security information, and data
encryption/decryption, etc.
“Digital family” is a technology trend and new business opportunity. The experts
consider that the basic need of digital family is convenient, diversified, and
low-error-rate remote control.
Both these examples described above present
opportunities for electro-optics modulation technology. The optical modulator, in this
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Journal of China University of Science and Technology Vol.50-2012.01
paper is often used as the aperture of the lens in the cell phone camera. We found that it
could also be used as the optical modulator for optical coding and integrate the digital
coding technology to develop a digital coding system. The purpose of this research,
through experiment, is conducting a study of “the near-infrared electro-optics
modulation coding technology”, and then to complete a small “near-infrared
electro-optics modulation coding system”, which can accomplish the modulation
/demodulation and encoding/decoding of optical data Subject to the availability of low
loss, low dispersion and wide-band (0.8-1.2μm) near-infrared optical fiber, we can also
create near-infrared optical fiber sensing, near-infrared digital information transmission,
near-infrared electro-optical biochemical detection, and other related technologies.
II. Modulation Code Principle [7]
The generation of a pulse width modulated signal requires that the amplitude of the
input analog signal be sampled periodically. The sampled amplitude is then converted
into a pulse width. For purposes of illustration, Fig. 1 depicts a particular procedure
wherein both the leading and trailing edges are modulated. The amplitude of the
triangular wave is slightly greater than the peak-to-peak excursion of the signal itself.
This method results in a natural sampling in which the message wave varies during the
sweeping process, and the length of a width modulated pulse is proportional to the
magnitude of the message wave at the pulse edges.
A double Fourier series method is used to trail edge modulated by natural samples of
a sinusoidal modulating wave:
F (t ) 

sin(ms t )   J n (m M )
1
n
S (t )  
 
sin(mc t  ns t  m 
)
2
m
m
2
m 1
m 1 n 
where the message signal is
S (t )  M cos(s t )
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The Research of Near-Infrared Electro-Optics Modulation Coding Technology
And the carrier is assumed to be saw-tooth waveform with the repetition rate of ωc/2.
By symmetric argument, the spectra of pulse width modulated pulses with both edges
modulated can be obtained easily from equation (1) :
M p (t ) 
1



S (t )  4 (1) m  k
m 1 k  0
J 2 k 1 (m M )
 [cos(mc t ) cos((2k  1)s t )]
m
Equation (3) represents a fundamental advantage of both edge modulated pulses with
natural samplings. The harmonic interference (the second term in equation (1)) is
completely suppressed, and all the even order sidebands of the modulated signal around
the carrier frequency are also eliminated. This will reduce the crosstalk between
adjacent multiplexed channels.
In general, in order to faithfully reproduce the original signal, we require
c  2s
Therefore, PWM will be free from intermodulation as long as the Fourier
components of (ωc - nωs) in equation (3) are negligible for n > 3. The conditions will be
easily met if the modulation index M is small, since higher order Bessel functions
vanish for small arguments.
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Journal of China University of Science and Technology Vol.50-2012.01
S(t)
Slicing
Circuit
＋
Triangular Waveform
Generator
(a)
Fig. 1 Generation of pulse width modulated pulses: (a) block diagram, (b) summation of
signal and carrier, (c) PWM output.
For a multi-channel system, M is inevitably small so that the interference of the
intermodulation products with the baseband signal will be small. It should be noted,
however, that as M decreases, the signal-to-noise ratio SNR will proportionally suffer. It
is interesting to compare equation (3) with typical frequency modulated (FM) signal
spectra:

M f (t )  const  J n (
n 
c
s
) cos[(c  ns )t 
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n
]
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The Research of Near-Infrared Electro-Optics Modulation Coding Technology
As anticipated, there exists a considerable similarity between equation (3) and
equation (5). It should be noted, however, that equation (5) contains the even order
sidebands of the modulated signal around the carrier frequency, while they are missing
in equation (3)
III. Near-Infrared Electro-Optics Modulation Coding System
The microprocessor (8051) sends signals to trigger Optical Modulator Device in
order to change the state of aperture. The different state of aperture produces different
optical signal. Through the output of the photodiode, optical signal is transformed to
different analog electronic signal and sent back to 8051 for signal processing. [8] The
light source here, with the photodiode circuit, can become an electro-optical switch.
Optical signals through the photodiode are converted into electronic signals. Through
the A/D converter, the electronic signals are sent to 8051 and then used to complete
signal processing as well as internal database comparison after software operation to
verify the optical signal mode. In this paper, we assume the number of aperture types is
“x”, and the number of components (or duty cycle) per signal is “y”. Then we can get
the number of modes by taking “x” to the power “y”.
The number of Modes:
xy
(6)
x：the number of aperture types
y：the number of components per signal
We develop a specific technology to build the near-infrared electro-optics
modulation coding system; this modulation coding system applies to NIR region which
is not visible to the human eye. The system in Fig.2 is composed of the followings: (1)
the NIR fiber unit as shown in Fig.2, (2) the electro-optical device (include transmitter
and receiver), (3) the optical modulator device and (4) 8051 single chip micro-controller
unit (include data processor and LCD display). The NIR fiber unit includes input fiber,
collimator, collective lens, and output fiber. The electro-optical device includes light
emitting diode (LED) source, current driver, photodiode, amplifier and comparator. The
optical modulator device includes three apertures (large aperture, small aperture and one
shutter) and two solenoids. The LED of transmitter emits the NIR light which passes
through the collimator lens and becomes a collimated beam. The collimated beam
passes through the optical modulator device. The modulation coding beam is focalized
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Journal of China University of Science and Technology Vol.50-2012.01
by collective lens of the receiver and be detected by the photodiode. The light source of
transmitter has NIR LED which is modulated by electric signal and enters the NIR fiber
coupled by the lens. The LED will be lighted by the pulse-modulated signal.
Fig.3 shows the pulsed LED driver circuit. The circuit uses three CMOS
inverters. It is amplified by the following Darlington pair. The LED will be lighted by
the pulsed signal.
Data Line
8051
Power
supply
A/D
Analog signal
Transmitter
Collimator
lens
Receiver
Input
fiber
Collimated
beam
Control line
Collective
lens
Optical
Modulator
Device
Output
fiber
Fig.2 Block diagram of near-infrared electro-optics modulation coding system
The following equation shows the resonant frequency: f
f
(7)
1
 0 . 4 0 52 R
2R C
0 . 6
1  R1 R 2

9 3

Darlington pair
LED driver
circuit
C
R1
R2
Fig.3 The pulsed LED driver circuit
where R1, R2 are resistors and C is capacitor.
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The Research of Near-Infrared Electro-Optics Modulation Coding Technology
The pulse-modulated light can also prevent the interference of lights from the
environment, because pulse-modulated light is different from ambient light due to its
pulse frequency. If the lens is used to collect the light into the fiber, we can derive the
following equation:[9-10]
ET

LS


d 
4f /# 1 T 
dS 

(8)
2
ET：the irradiance of the fiber (W/cm2)
LS：the radiance of the LED source (W/cm2．sr)
τ：transmittance of the lens
dS：the distance between LED source and lens
dT：the distance between lens and fiber
f/#：the ratio of the effective focal length to pupil diameter of the lens
Once the electro-optics modulation coding signal is created, the detector measures
the signal. The most commonly used detector is the photodiode whose property is the
incident light is linearly proportional to the output current. The photodiode generates the
electron after it is hit by the light. Because the measurement is the variation of the
voltage, the output current should be converted into a voltage signal. At both
photoconductive (PC) and photovoltaic (PV) modes, the circuit can be connected with
the external resistors or amplifier to convert the current into a voltage signal. The
electrical signal generated from the photodiode is very small. Due to very small output
voltage, such as mV or μV, the OP amplifier, LM324M, is used to amplify the small
signal into large signal. In the PV mode, there is a reverse bias. Due to the reverse bias
being applied to the photodiode, the electromagnetic (EM) waves in the ambient
environments will have less effect on the photodiode. To further avoid the EM waves,
we use a metal shield to decrease the EM interference in the circuits.
IV. Experiments and Results
The specifications of devices used for the experiment are listed in Table 1. The
experiment parameters are as follows. The photogrph of the near-infrared electric-optics
modulation coding system is shown in Fig.4. The LED trigger duration is 46μs to prevent
from being interfered with visible light. These aperture diameters are 2mm and 0.05mm.
The aperture trigger duration is minimized to 10 ms. First, optical modulation used to
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Journal of China University of Science and Technology Vol.50-2012.01
be calibrated, and then the electro-optics modulation coding system is tested. The NIR
fiber unit and optical modulator are placed on a small precision optical bench.
Experiment procedure which is separated into two parts is described as follows:
Fig.4 The photogrph of the near-infrared electric-optics modulation coding system
Table 1 Specifications of the Device
Specifications
Photodiode
Responsivity,R
Dynamic range
Noise
Steady current source
NIR fiber N.A.
The wavelength of LED at the peak irradiance
Core diameter(μm)
Collimated beam size
Solenoid
Micro processor
LCD display
Environental intensity of illumination
Si Photo detector
0.4 A/W(λp=780nm)
1.36~2.98V
Dark current ~10nA
Auto power control
0.46
950 nm
740
5 mm
DC 3V@273mA
AT89C51
Format Char. x line:
16×2
550 lux
The first part: collimator optical axis alignment as indicated in Fig. 5. [11]
(1) The system calibration is done by placing light source near the focus of the lens
under test and a plane mirror is placed in front of the lens so as to reflect the light
backing to the lens.
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The Research of Near-Infrared Electro-Optics Modulation Coding Technology
(2) The light source and lens is fine tuned until the reflected beam from the plane
mirror being focused on the position of the light source.
(3) The photodiode is also calibrated and fine tunes as in step 1 & 2, but to achieve
maximum signal receipt.
plane mirror
Collimator lens
p
Collimator
bean
optical axis
f
p: NIR fiber exit
f: focal length
Fig.5 Collimator optical axis alignment
The second part: system operation
(1) Build the standard database.
(2) Confirm the three states -large aperture, small aperture, and close (shutter) state
with correct corresponding mode as shown in Fig.6. If the state has any mistake,
adjustment would need to be made before proceeding to the next step.
(3) Every mode must be tested automatically for 65536 times and the error rate of the
corresponding mode is recorded. The result is displayed on the LCD.
In equation (7), Y is the number of components per signal, where three are used in
this experiment designated as trigger1, 2, and 3. These components can trigger in turn.
X is the number of aperture type where the three aperture types are  for large
aperture,  for small aperture and  for close aperture. The three components and
three apertures allow having 27 kinds of coding modes in this system. The test results
are shown in Fig.7. The communication error rate of the modulation coding system is
less than 0.2%. The error that occurred in the system can easily be corrected with
standard digital data check software.
large aperture (  )
small aperture (  )
Fig.6 Three aperture states
86
close (  )
Journal of China University of Science and Technology Vol.50-2012.01
error rate of 27 modes
error rate (%)
0.2
0.15
0.1
0.05
0
1
3
5
7
9
11 13 15 17 19 21 23 25 27
mode type
Fig.7 Error rate of 27 modes
V. Conclusion
The technology combines optical-modulation and electro-modulation and the former
is convenient to expand more modes. Its data transmission is safe, stable, and secure.
The technology can be expanded into the application area of information and
communication system on account of the large dynamic range of light signal encoding.
This innovative technology is a forward-looking and practical way for “information and
communication” technological area.
VI. Acknowledgments
The authors would like to give a great thank to National Science Council
(NSC96-2622-E-023-CC3) in Taiwan which offer funds to us and enable us to study
carefree.
VII.
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