Chapter 2 The Concept of New Optical Modulation System
2.5 The new proposed model of optical modulation system
In section 2.3.2, there is an introduction of three traditional modulation schemes to generate optical RF signal. In this work, we propose a new modulation approach to generate optical vector signals by frequency multiplication based on a DSBCS scheme and only using a single-electrode MZM. Fig. 2-6 schematically depicts the optical vector signal generation system. The single-electrode MZM driving signals consist of a sinusoidal signal of frequency f1 modulated with a RF signal and a sinusoidal signal of frequency f2, as indicated in insets (a) and (b) of Fig. 2-6, respectively. To realize the DSBCS modulation scheme, the MZM is biased at the null point.
Inset (d) in Fig. 2-6 presents the generated optical spectrum that has two upper-wavelength sidebands (USB1, USB2) and two lower-wavelength sidebands (LSB1, LSB2) with carrier suppression.
At the base station, the LSB2 subcarrier is filtered out for the upstream data link (inset (f)), and the rest of the signal is sent to local users. After square-law photo detection, the cross term of USB2 and USB1 generates the RF signal at the difference frequency (f2-f1). Concurrently, the cross term of LSB1 and USB2 yields the RF signal at the sum frequency (f2+f1). Here we only consider the RF signal at the sum frequency and a frequency multiplication of optical RF signal can be achieved by proper choosing the frequencies of vector signal and sinusoidal signal. The filtering out of LSB2 subcarrier not only provides an upstream light source but also eliminates performance fading. As presented in Fig. 2-7, a two-tone RF single at frequencies of 5 GHz and 10 GHz is used to simulate the performance fading.
After square-law photo detection, the generated RF signals at difference and
sum frequencies of 5 GHz and 15 GHz suffer performance fading before filtering. If an optical filter is utilized to remove anyone of the four optical subcarriers, the fading of both 5-GHz and 15-GHz RF signals can be readily eliminated. Notably, a frequency multiplication (1.5 times after square-law photo detection) scheme is adopted to reduce the cost of the electronic components, especially for RF signals in the millimeter-wave range.
Figure 2-6 Concept of the proposed system. (LD: laser diode, MZM:
Mach-Zehnder modulator, SMF: single mode fiber, C: circulator, FBG: fiber bragg grating, RSOA: reflective semiconductor optical amplifier)
w/o Filtering, 5-GHz RF signal w/o Filtering, 15-GHz RF signal Filtering LSB2, 5-GHz RF signal Filtering LSB2, 15-GHz RF signal
0 20 40 60 80 100
Figure 2-7 Simulation of RF performance fading versus SMF transmission length.
Chapter 3
The theoretical calculations of proposed system
3.1 Introduce MZM
For MZM with configuration as Fig. 3-1, the output E-filed for upper arm is EU E · a · e∆φ (1)
∆φ π· π (2)
∆φ is the optical carrier phase difference that is induced by v , where a is the power splitting ratio.
The output E-filed for upper arm is
EL E · √1 a · e∆φ (3)
∆φ is the optical carrier phase difference that is induced by v
∆φ π· π (4) The output E-filed for MZM is
ET E · a · b · e∆φ √1 a · √1 b · e∆φ (5) where a and b are the power splitting ratios of the first and second Y-splitters in MZM, respectively. The power splitting ratio of two arms of a balanced MZM is 0.5. The electrical field at the output of the MZM is given by
ET · E · e∆φ e∆φ (6) ET E · cos ∆φ ∆φ · exp j∆φ ∆φ (7) For single electro x-cut MZM. The electrical field at the output is given by EOUT E · cos ∆φ ∆φ · exp j∆φ ∆φ (8) Add time component, the electrical field is
EOUT E · cos ∆φ · cos t (9) where E0andωcdenote the amplitude and angular frequency of the input optical carrier, respectively; V t is the applied driving voltage, and ∆φ is the optical carrier phase difference that is induced by between the two arms of the MZM. The loss of MZM is neglected. consisting of an electrical sinusoidal signal and a dc biased voltage can be written as,
cos t (10) where is the dc biased voltage, and are the amplitude and the angular frequency of the electrical driving signal, respectively. The optical carrier phase difference induced by is given by
∆φ Vπ Vπ ·π (11) Eq. (10) can be written as:
EOUT E · cos V V cos ωt
Vπ ·π
2 · cos ω t E · cos b m · cos ωRFt · cos ω t E · cos ω t · cos b · cos m · cos ωRFt
sin b · sin m · cos ωRFt (12) where V π is a constant phase shift that is induced by the dc biased voltage, and V π is the phase modulation index.
cos x sin θ J x 2 J x cos 2nθ
∞
sin x sin θ 2 J x sin 2n 1 θ
∞
cos x cos θ J x 2 1 J x cos 2nθ
∞
sin x cos θ 2 1 J x cos 2n 1 θ
∞
(13) Expanding Eq. (12) using Bessel functions, as detailed in Eq. (13). The electrical field at the output of the MZM can be written as:
EOUT E · cos ·
cos · 2 · 1 · m · cos 2
∞
sin · 2 · 1 · · cos 2 1
∞
(14) where is the Bessel function of the first kind of order n. the electrical field of the mm-wave signal can be written as
EOUT E · cos · J m · cos ω t
E · cos · · cos
∞
2 π
E · cos · · cos 2
∞
E · sin · · cos
∞
2 1
E · sin · · cos
∞
2 1
(15)
Figure 3-1 The principle diagram of the optical mm-wave generation using balanced MZM.
3.2 Theoretical calculation of single drive MZM 3.2.1 Bias at maximum transmission point
When the MZM is biased at the maximum transmission point, the bias voltage is set at 0, and cos 1 and sin 0. Consequently, the electrical field of the mm-wave signal can be written as
· · cos
· · cos
∞
2
· · cos 2
∞
(16)
The amplitudes of the generated optical sidebands are proportional to those of the corresponding Bessel functions associated with the phase modulation index . With the amplitude of the electrical driving signal equal to , the
maximum is . As 0 , the Bessel function for 1 decreases and increases with the order of Bessel function and m, respectively, as shown in Figure 3-2. , , , and are 0.5668, 0.2497, 0.069, and 0.014, respectively. Therefore, the optical sidebands with the Bessel function higher than can be ignored, and Eq. (14) can be further simplified to
· · cos
· · cos 2 · · cos 2 · · cos 4
· · cos 4 (17)
0 2 4 6 8 10
-0.5 0.0 0.5 1.0
m
J0 J1 J2 J3
Figure 3-2 The different order of Bessel functions vs. m.
3.2.2 Bias at quadrature point
When the MZM is biased at the quadrature point, the bias voltage is set at
V , and cos √ and sin √ . Consequently, the electrical field of the mm-wave signal can be written as
EOUT 1
√2· E · J m · cos ω t 1
√2· E · J m · cos ω ωRF t 1
√2· E · J m · cos ω ωRF t 1
√2· E · J m · cos ω 2ωRF t π 1
√2· E · J m · cos ω 2ωRF t π 1
√2· E · J m · cos ω 3ωRF t π 1
√2· E · J m · cos ω 3ωRF t π
(18) 3.2.3 Bias at null point
When the MZM is biased at the null point, the bias voltage is set at V , and cos 0 and sin 1. Consequently, the electrical field of the mm-wave signal using DSBCS modulation can be written as
EOUT E · J m · cos ω ωRF t E · J m · cos ω ωRF t E · J m · cos ω 3ωRF t π E · J m · cos ω 3ωRF t π E · J m · cos ω 5ωRF t
E · J m · cos ω 5ωRF t (19)
3.3 Theoretical calculations and simulation results 3.3.1 The generated optical signal
The theoretical calculations of proposed system, the driving RF signal consisting of an electrical sinusoidal signal and a dc biased voltage can be written as
cos t cos t (20)
where is the dc biased voltage, , and , are the amplitude and the angular frequency of the electrical driving signals, respectively. The optical carrier phase difference induced by is given by
· cos
V cos t cos t
· cos cos t cos t (21)
where is a constant phase shift that is induced by the dc biased voltage, and V , V is the phase modulation index.
· cos · cos cos t cos t · sin · sin cos t cos t
· cos cos m cos cos cos sin cos sin cos
· sin sin cos t cos cos t
cos cos t sin cos t (22) When the MZM is biased at the null point, the bias voltage is set at V , and cos 0 and sin 1. Consequently, the electrical field of the mm-wave signal using DSBCS modulation can be written as
sin cos t cos cos t
cos cos t sin cos t (23) First, to expand equation sin cos t cos cos t
sin cos t cos cos t
2 1 J cos 2n 1 t
∞
· J 2 1 J cos 2n t
∞
2J cos t 2J cos 3 t
· J 2J cos 2 t 2J cos 4 t (24) The optical sidebands with the Bessel function higher than can be ignored. Consequently, the electrical field can be written as
sin cos t cos cos t
2J J cos t
2J J cos 3 t
4J J ·1
2 cos 2 t cos 2 t
4J J ·1
2 cos 3 2 t cos 3 2 t
(25) Add time component cos t
sin cos t cos cos t cos t
2J J cos t cos t
2J J cos 3 t cos t
4J J ·1
2 cos 2 t cos 2 t cos t
4J J ·1
2 cos 3 2 t cos 3 2 t cos t
(26)
J J , 2J J , 4J J , and 4J J are
shown in Figure 3-3. Therefore, the optical sidebands with the Bessel function
4J J can be ignored, and Eq. (14) can be further simplified to
sin cos t cos cos t cos t
J J cos t cos t
J J cos 3 t cos 3 t
J J cos 2 t cos 2 t
J J cos 2 t cos 2 t
(27)
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 -0.1
0.0 0.1 0.2 0.3 0.4
m
1,m
2
J0J1 J0J3 J1J2 J2J3
Figure 3-3 The different order of Bessel functions vs. m.
Second: To expand equation cos cos t sin cos t
cos cos sin cos
2 1 cos 2
∞
· 2 1 cos 2 1
∞
2 cos 2 2 cos 4
· 2 cos 2 cos 3
2 cos
2 cos 3
4 ·1
2 cos 2 cos 2
4 ·1
2 cos 3 2 cos 3 2
(28) Add time component cos
cos cos sin cos cos
2 cos cos
2 cos 3 cos
4 ·1
2 cos 2 cos 2 cos
4 ·1
2 cos 3 2 cos 3 2 cos
cos cos
cos 3 cos 3
cos 2 cos 2
cos 2 cos 2 (29)
The output electrical filed can be rewritten as
cos sin cos cos cos
cos cos sin cos
t ·
cos cos
cos 3 cos 3
cos 2 cos 2
cos 2 cos 2
cos cos
cos 3 cos 3
cos 2 cos 2
cos 2 cos 2
(30)
Figure 3-4 Illustration of the optical spectrum at the output of the MZM.
( 6 )
The commercial software, VPI WDM-TransmissionMaker© 5.0, is used to simulate numerically the power ratio. Fig. 3-5 shows the optical power ratio (OPR, , ) as a function of MI. The , is defined as
,
(31) where and are the optical powers of the sideband frequency at
1 · 0 · and the sideband frequency at · · ,
respectively. As MI falls from one to zero, the optical power ratios are improved.
0.0 0.2 0.4 0.6 0.8 1.0
0 10 20 30 40 50 60 70 80
MI
Theoretical P
10,+21,P
10,+12Numerical P
10,+21,P
10,+12Theoretical P
10,30,P
10,03Numerical P
10,30,P
10,03Power (10dB/Div.)
Figure 3-5 Illustration of the optical spectrum at the output of the MZM.
3.3.2 The generated electrical signal
After square-law detection using an ideal PD with responsivity R, the photocurrent can be expressed as
· | | (32)
The RF signal is E ·
J J 2J J J 2J J 4J J J
· cos 2 t
2J J J J 2J J J J
2J J J J 2J J J J
2J J J J 2J J J J
· cos t cos t
J J 2J J J 2J J
4J J J · cos 2 t
2J J J J 2J J J J
2J J J J
· cos 3 t cos 3 t
2J J J 2J J J
2J J J J 2J J J J
· cos 2 2 t
2J J J 2J J
· cos 4 t
J J 2J J J J
· cos 2 4 t 2J J J J · cos 5 t RF
(33)
Figure 3-6 Illustration of the electrical spectrum of generated BTB mm-wave signals using MZM after square-law PD detection. ( 6 )
Fig. 3-7 shows the RF signal power ratio (RFPR, , ) as a function of MI.
The , is defined as
,
(34) where and are the RF signal powers of the frequency at 1 ·
1 · and the frequency at · · ,respectively. The RF signal power at sum and subtract frequencies are the same, so do not consider the , term.
0.0 0.2 0.4 0.6 0.8 1.0 Figure 3-7 The RF signal power ratio (RFPR, , ) as a function of MI.
Table 3-1 Measure the RF power without optical filtering
Frequency Amplitude
2 J J 2J J J 2J J
4J J J
, 2J J J J 2J J J J
2J J J J 2J J J J
2J J J J 2J J J J
2 J J 2J J J 2J J
4J J J
3 , 3 2J J J J 2J J J J
2J J J J
2 2 2J J J 2J J J
2J J J J 2J J J J
4 2J J J 2J J
2 4 J J 2J J J J
5 , 5 2J J J J
3.3.3 Consider dispersion effect
When optical RF signals are transmitted over a single-mode fiber with dispersion, a phase shift to each optical sideband relative to optical carrier is induced. The propagation constant of the dispersion fiber can be expressed as
(35)
where is the derivative of the propagation constant evaluated at . The effect of high order fiber dispersion at 1550-nm band is neglected. For carrier tones with central frequency at , we have
(36)
and
· D (37)
where c is light speed in free space, D is the chromatic dispersion parameter, and is the frequency of the optical carrier. For a standard single-mode fiber, D is 17-ps/(nm.km). The fiber loss is ignored. Therefore, after transmission over a single-mode fiber of length z, the electrical field can be written as
· cos 1
2
cos 1
2
cos 1
2
cos 1
2
(38)
After square-law photo detection, the RF signal can be expressed as
· DC RF
2 · cos 1
2 · cos 1
2
2 · cos 1
The RF signal at the substrate frequency :
I J J J J · The RF signal at the substrate frequency can be written as
I J J J J · cos c d cos c d
The RF signal at the sum frequency :
I J J J J ·
cos t 1 2
1 2
cos t 1
2
1 2
(43) Define
e t
f 1
2
1 2
1 2
(44) The RF signal at the sum frequency can be written as
I J J J J · cos e f cos e f
I 2J J J J · cos e cos f
2J J J J · cos 1 2 · cos t
(45)
Fig. 3-8 and Fig. 3-9 show the numerical simulation and theoretical solutions, the RF fading problem with the same results. Fig. 3-8 RF signal is driving 5 GHz and 30 GHz sinusoidal. Fig. 3-9 shows the results after 50km SMF transmission.
0 10 20 30 40 50 -50
-40 -30 -20 -10 0 10
Transmission Length (km) 25G Numerical
35G Numerical Theoretical
Power (10dB/Div.)
Figure 3-8 shows the numerical and theoretical solution for RF signal fading
issue after transmission.
30 35 40 45 50 55 60
-50 -40 -30 -20 -10 0 10
Frequency (GHz) Numerical
Theoretical
Power (10dB/Div.)
Figure 3-9 RF power vs. .
3.4 The generated optical signal using optical filtering 3.4.1 Analysis of the generated signal
When we use fiber grating, the equation for the optical spectrum is
·
J J · cos t cos t
J J cos 3 t cos 3 t
J J cos 2 t cos 2 t
J J cos 2 t cos 2 t
J J · cos t cos t
J J cos 3 t cos 3 t
J J cos 2 t cos 2 t
J J cos 2 t cos 2 t (46)
Figure 3-10 shows the optical spectrum when the LSB1 is filter out.
Figure 3-11 shows the optical spectrum when the LSB2 is filter out.
After square-law detection, the photocurrent can be expressed as
E ·
· J J 1 · J J J
2J J 2 2 · J J J
· cos 2 t
1 · J J J J
1 · J J J J 1 · J J J J 2J J J J
2J J J J 2J J J J
· cos t
· J J J J
1 · J J J J 1 · J J J J 2J J J J
2J J J J 2J J J J
· cos t
· J J 1 · J J J
2J J 2 2 · J J J
· cos 2 t
1 · J J J J
1 · J J J J 2J J J J
· cos 3 t cos 3 t
1 · J J J 1 · J J J
2J J J J 2J J J J
· cos 2 2 t
1 · J J J 2J J
· cos 4 t
J J 2J J J J
· cos 2 4 t 2J J J J · cos 5 t RF
(47)
Table 3-2 Measure the RF power with optical filtering
Frequency Amplitude
2 · J J 1 · J J J
2J J 2 2 · J J J
1 · J J J J
1 · J J J J
1 · J J J J
2J J J J
2J J J J
2J J J J
· J J J J
1 · J J J J
1 · J J J J
2J J J J
2J J J J
2J J J J
2 · J J 1 · J J J
2J J 2 2 · J J J
3 , 3 1 · J J J J
1 · J J J J
2J J J J
2 2 1 · J J J
1 · J J J
2J J J J
2J J J J
4 1 · J J J 2J J
2 4 J J 2J J J J
5 2J J J J
Figure 3-12 Illustration of the electrical spectrum of generated BTB mm-wave signals when the LSB1 is filter out. ( 6 )
Figure 3-13 Illustration of the electrical spectrum of generated BTB mm-wave
signals when the LSB2 is filter out. ( 6 )
3.4.2 The effects of fiber dispersion
Consider fiber dispersion effect, the electrical field can be written as
· · J J cos ω t 1 After square-law photo detection, the RF signal can be expressed as
E · DC RF
The RF signal at the substrate frequency : The RF signal at the substrate frequency can be written as
I J J J J The RF signal power at the substrate frequency can be written as
P J J J J · A B 2AB · cos θ
The RF signal at the sum frequency
The RF signal at the sum frequency can be written as
I J J J J
The RF signal power at the sum frequency can be written as
P J J J J · A B 2AB · cos θ
J J J J · 2 cos 1
2 2 2 cos 1
2 · · cos θ
(57) Or
P J J J J · A B 2AB · cos θ
J J J J · 2 cos 1
2 2 2 cos 1
2 · · cos θ
(58) Because grating only filter out LSB1 or LSB2 at the same time, so the equation of the RF signal power at the sum and substrate frequency are the same. The RF signal power can be written as
P P
J J J J · 2 cos 1
2 1
2 2 cos 1
2 · 1 · cos θ
(59) If grating is filter out 28dB, 20Log 28, 0.04.
0 10 20 30 40 50 -50
-40 -30 -20 -10 0 10
Transmission length (Km)
Numerical 25GHz Numerical 35GHz Theoretical
Pow er (10dB/Div. )
Figure 3-14 shows the RF signal power vs. transmission length when the LSB1
is filter out.
0 10 20 30 40 50
-50 -40 -30 -20 -10 0 10
Transmission length (Km)
Numerical 25GHz Numerical 35GHz Theoretical
Pow er (10dB/Div. )
Figure 3-15 shows the RF signal power vs. transmission length when the LSB2
is filter out.
3.5 The optimal optical power ratio condition 3.5.1 Signal without optical filtering
We assume the output e-filed of LSB2, LSB1, RSB1 and RSB2 are E , E , E and E respectively. When we use single drive MZM and set the bias point of MZM at Vπ and then the e-filed is
E E , E E (60)
For the PSK signal that the total power can be written as
E E E E P
2E 2E P (61)
The optical field is then detected using and ideal square-law photodetector, and the RF signals generated are mathematically evaluated as follows:
2E 2E E · E E · E E · E E · E
The f f RF signal term is
E · E E · E 2E · E (62)
Where E P E for PSK signal The RF signal become
2E · P E (63)
The maximum of RF signal power originates from
2E · P E
′
0 (64)
To solve the differential equation we would get the e-filed E √P 2⁄ , E √P 2⁄
E : E : E : E 1: 1: 1: 1 (65)
The optical powers are
I I I I P 4⁄ , I : I : I : I 1: 1: 1: 1 (66)
For the OOK signal that the total power can be written as
2E E E 2E P
4E 2E P (67)
The f f RF signal term is
E · E E · E 2E · E (68)
Where E P E for OOK signal The RF signal become
2E · P E (69)
The maximum of RF signal power is happened in
2E · P E
′
0 (70)
To solve the differential equation we would get the e-filed E √P 2⁄ , E P 8⁄
and
E : E : E : E 1: √2: √2: 1 (71)
The optical powers are I I 2E P 4⁄
I I E P 4⁄ (72)
and
I : I : I : I 1: 1: 1: 1 (73)
When the optical signal without filtering, the optimal SOPR of both PSK and OOK RF signals is 0-dB.
Figure 3-16 shows optical spectrum without fiber grating filter out.
Figure 3-17 the e-filed power for BPSK signal between zero and one.
Figure 3-18 the e-filed power for OOK signal between zero and one.
3.5.2 When the LSB2 or LSB1 is filter out
Using fiber grating to remove LSB1 and getting maximum output RF signal condition for PSK signal is
E : E : E 1: 1: √2 (74)
The optical power equal
I : I : I 1: 1: 2 (75)
and for OOK signal
E : E : E 1: 1: 1 (76)
The optical power equal
I : I : I 1: 1: 2 (77)
The optimal SOPR of both PSK and OOK RF signals is 3-dB when the LSB2 with filtering.
Using fiber grating to remove LSB1 and getting maximum output RF signal condition for PSK signal is
E : E : E 1: √2: 1 (78)
The optical power equal
I : I : I 1: 2: 1 (79)
And for OOK signal
E : E : E 1: 2: 1 (80) The optical power equal
I : I : I 1: 2: 1 (81)
The optimal SOPR of both PSK and OOK RF signals is -3-dB when the LSB1 with filtering.
Figure 3-19 shows optical spectrum when the LSB2 is filter out.
Figure 3-20 shows optical spectrum when the LSB1 is filter out.
Chapter 4
Experimental demonstration of the proposed system
4.1 preface
In chapter 3, we provide the theoretical and numerical results for the concept of proposed system. Therefore, the result can be tried to apply to the radio-over-fiber system. In this chapter, we will build the experimental setup for the propose system based on DSBCS modulation. Figure 4-1 shows the optical spectrum for the fiber Bragg grating reflection and transmission.
Figure 4-1 the optical spectrum for the fiber Bragg grating reflection and
transmission.
1555.65-20 1555.7 1555.75 1555.8 1555.85
-10 0
Reflection (dB)
Wavelength (nm)
1555.65 1555.7 1555.75 1555.8 1555.85-30
-20 -10 0
Transmission (dB)
Reflection Transmission
4.2 Experimental results for optical signal without optical filtering 4.2.1 Experiment setup
Figure 4-2 displays the experimental setup for optical vector signal generation and transmission using a single-electrode MZM. The continue wave laser source about 1550nm is generation using tunable laser. The laser source is then passed through a polarization controller to achieve output optical power is a maximum when the MZM is biased at full point. The OOK/BPSK signal is a 1.25-Gb/s pseudo random binary sequence (PRBS) signal with a word length of 231-1 and is up-converted using a 5-GHz sinusoidal signal (f1). The 625-MSym/s QPSK signal at 5-GHz is generated using an arbitrary waveform generator. Then the RF OOK/BPSK/QPSK signals are combined with a 10-GHz sinusoidal signal (f2). This combined RF signal is fed into single-electrode MZM and the MZM is bias the null point. The generated optical spectrum that has two upper-wavelength sidebands (USB1, USB2) and two lower- wavelength sidebands (LSB1, LSB2) with carrier suppression. The generated optical signal is amplified by EDFA and then filtered by an optical tunable filter with a bandwidth of 38GHz. The input EDFA optical power is fixed -20dBm. The power of optical RF signal which entered fiber is set to less than 0 dBm to reduce the effect of both fiber nonlinearity and dispersion changing the duty cycle of optical microwaves. After transmitted over standard single mode fiber (SSMF), the transmitted optical microwave signal is converted into an electrical microwave signal by a PIN PD with a 3 dB bandwidth of 38 GHz, and the converted electrical signal is amplified by an electrical amplifier. If we would measure the RF signal at the subtract frequency, the RF signal is then passed through the electrical bandpass filter at
5GHz. The center frequency of bandpass filter is 15GHz when we would measure the RF signal at the sum frequency. After the photo receiver, the optical signal generates two RF signals with a subtract frequency of 5 GHz (f2-f1) and a sum frequency of 15 GHz (f1+f2), respectively. Insets (i) and (ii) of Figure 4-2 show the receiver architectures of RF OOK/BPSK and QPSK signals, respectively. The RF OOK/BPSK signal is down-converted to baseband signal and directly tested by a BER tester. The RF QPSK signal is down-converted to 5 GHz by a 10 GHz oscillator and a mixer to realize intermediate frequency (IF) demodulation. A digital real-time oscilloscope (Tektronix DPO71254) stores the waveform and an off-line digital signal processing (DSP) program using Matlab is employed to demodulate the QPSK signal. For QPSK signal, the bit error rate (BER) performance is calculated from the measured modulation error ratio (MER). The MER is defined as MER I I I QQ Q , where Ir and Qr represent the demodulated in-phase and quadrature-phase symbols, and Io and Qo are the ideal normalized in-phase and quadrature-phase QPSK symbols. The optical intensity of the data-modulated subcarriers (5 GHz) relative to that of the sinusoidal subcarrier (10 GHz) can be easily tuned by adjusting the input electrical power to optimize the performance of the optical RF signals.
Figure 4-2 shows the experimental setup to receive sum frequency.
4.3.2 Optimal condition for RF signal
Fig. 4-3 and Fig. 4-4 show the optical spectrums in different SOPR (SOPR=
Ps/Pd, Ps and Pd are the optical powers of the 10-GHz subcarrier and the 5-GHz data-modulated subcarrier, respectively.) for BPSK and OOK signal. The BER curves of BPSK and OOK signal at substrate frequency shown in Fig. 4-5 and
Ps/Pd, Ps and Pd are the optical powers of the 10-GHz subcarrier and the 5-GHz data-modulated subcarrier, respectively.) for BPSK and OOK signal. The BER curves of BPSK and OOK signal at substrate frequency shown in Fig. 4-5 and