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Performance analysis of an OSSB RoF link using 90o & 120o Hybrid coupler
Optics Communications ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Performance analysis of an OSSB RoF link using 90o & 120o Hybrid coupler Parvin Kumar a, Sanjay Kumar Sharma a, Shelly Singla b a b
Krishna Institute of Engineering and Technology, Ghaziabad (U.P.)-201206, India Indus Institute of Engineering and Technology, Kinana (Haryana)-126114, India
art ic l e i nf o
a b s t r a c t
Article history: Received 23 July 2015 Received in revised form 14 September 2015 Accepted 22 September 2015
This paper presents an analysis, simulation and comparison of the performance of Optical single sideband (OSSB) radio over fiber (RoF) system based on a dual drive Mach Zehender modulator (DD-MZM) using 90o and 120o hybrid coupler including the effects of phase noise from RF signal oscillator and laser source, input RF signal power and fiber dispersion. Signal to Noise ratio (SNR) is significantly influenced by phase noise from RF signal oscillator and input RF signal power. On comparison with conventional 90o hybrid coupler system, the performance of the considered system improves by 5.64 and 0.67 dB in terms of SNR, when RF and laser phase noises are increased. & 2015 Elsevier B.V. All rights reserved.
Keywords: Optical single sideband (OSSB) Radio over Fiber (RoF) Dual drive Mach Zehender modulator (DDMZM) phase noise from a laser and an RF oscillator and Signal to Noise ratio (SNR)
1. Introduction A Radio over Fiber (RoF) system is a promising technique for next generation wireless networks as it has capability to achieve multi-gigabit per second data rate to support bandwidth intensive applications. Optical single sideband (OSSB) modulation has become an attractive technology for achieving long distance RoF based transmission highly accurate optical sensing [1–3], highresolution optical vector network analysis [4–6], optical wavelength conversion [7], and optical coherence tomography [8]. The OSSB signal is not in affected by the errors induced by fiber dispersion, which is severe in the conventional double sideband (DSB) modulated RoF transmission system. OSSB modulation can be implemented using various schemes [9,10]. Majority two schemes are preferred optical filtering and 90° phase shift procedure. In the optical filtering method, the OSSB signal is generated by removing one of the first order sidebands in an intensity modulated or phase modulated signal. While the OSSB modulation signal generated using a 90° phase shift method is almost independent of the wavelength of the optical carrier. In this method, two RF signals with a phase difference of 90° are fed to a dual drive Mach Zehender modulator (DDMZM) [11,12]. The signal in the upper and lower arms of the MZM have a phase difference of nπ/2 −∅0 , where nrepresents the order E-mail addresses: [email protected] (P. Kumar), [email protected] (S. Kumar Sharma), [email protected] (S. Singla).
of the sideband and ∅0 is the phase difference introduced by the dc bias. When ∅0=−π/2 or π/2, one of the first order sidebands would be cancelled out when the signals in the upper and lower arms are combined at the output port of the modulator. Thus, OSSB modulated signal is generated. When ∅0=−π/2, the 1st and þ3rd order sidebands are effectively suppressed. But the unwanted þ2nd and higher order sidebands still exist. The existence of the þ2nd order sideband causes power degradation due to the fiber dispersion. To overcome these problems it is desirable to eliminate 2nd order sideband. Keeping this in view, a novel OSSB modulation based on a 120° hybrid coupler and a DD-MZM is proposed and demonstrated by [13]. A RoF system suffers from various performance degradation parameters like phase noise, input RF signal power and high frequency band. It is highly desirable to evaluate performance of the system including all these errors. Thus, in this paper analysis has been done against these parameters for 90° & 120° hybrid coupler based systems. Simulations have also been carried out to evaluate the performance and draw comparison between them.
2. System Model Fig. 1.1 shows the implementation of OSSB RoF system based on a DD-MZM. With a proper dc bias, the phase difference between optical signals in the upper and lower arms of the DD-MZM is controlled. In a 90o phase shift method, the phase difference between the optical components is nπ/2- ∅0 while for 120o hybrid
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Please cite this article as: P. Kumar, et al., Optics Communications (2015), http://dx.doi.org/10.1016/j.optcom.2015.09.081i
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2
RF oscillator
V 90 o( 0, t) =
Optical Fiber Laser Diode
Electrical Spectrum Analyzer
Photodetector
DD-MZM DC Bias ( ) Optical Path
m =∞ ⎤ ⎧ ⎫ ⎪ ⎪ +⎨ ∑ ( j)mJm ( β)expj(wlt+Φl(t) + mwr t+mΦr( t)+m π )⎬⎪⎥⎥ ⎪ 2 ⎭⎦ ⎩ m =−∞
Electrical Path
Fig. 1.1. Block diagram of OSSB RoF system based on a dual drive Mach Zehender modulator.
coupler, the phase difference is nπ/3-∅0 . ∅0=π /3, the -1 and þ2 order sideband are simultaneously suppressed because these sidebands have a -180o and 180o phase difference respectively and are destructively inferred when combined at the output port of the DD-MZM. As a result, an OSSB signal with both of the -1st order sideband and þ2nd order sideband suppressed is generated. Similarly, when ∅0= − π /3,an OSSB signal with both of the þ 1st order sideband and -2nd order sideband suppressed is generated [13]. The signal from the laser diode and the RF oscillator are modeled as: st
nd
xl( t) = A 0expj( wlt+Φl(t))
(1)
V120 o( 0, t) =
V( 0, t) =
⎛ = A 0. L mod. ⎜ ⎝
l
l
l
r
r
V120 o( 0, t) ⎛ = A 0. L mod. ⎜ ⎝
⎛ 3 π⎞ J ( β)expj⎜ wlt+Φl( t) + ⎟ − ⎝ 2 0 6⎠
3 J ( β)expj wlt+Φl( t)+ w t+Φr( t) r 2 1
(
)
⎛ 3 π ⎞⎞ J ( β)expj⎜ wlt+Φl( t)+ 2w t + 2Φr( t) − ⎟⎟ r ⎝ 2 2 2 ⎠⎠
(9)
After the transmission over distance of L fiber km on optical fiber, the signals are modeled as: ⎛ α fiberL ⎞ fiber ⎟ −⎜ ⎧ 20 ⎝ ⎠⎨ J ⎩ 0
V90o( L, t)=A . L mod. L add. 10 0
π ⎟⎞ − 4⎠
⎛
( β). expj⎝ wlt ⎜
(
2 J1( β).expj wlt
⎛ +2.J2 ( β). expj⎜ wlt + 2wrt ⎝ +Φl( t−τ−)+2Φr ( t−τ−) − ϕ3 −
(3)
πVr 2 Vπ
.
0
θ and γ as ( 2π , 21 ) and (2 3π , 31 ), the OSSB
+Φl( t−τ0) − ϕ1 +
π ⎟⎞ 4⎠
(10)
(4)
⎡ ⎤ A 0. L mod. ⎧ π ⎨ expj⎢ wlt+ + Φl( t)+βcos(wrt+Φr (t))⎥ ⎣ ⎦ ⎩ 3 2 ⎡ 2π ⎤⎫ + expj⎢ wlt+Φl( t)+βcos(wrt+Φr ( t) + )⎥⎬ ⎣ 3 ⎦⎭
(5)
Using the above equations and defined functions, the output of DD-MZM can be represented in terms of Bessel function as:
⎩
π ⎞⎟ − 6⎠
⎛ 3 J ( β). expj⎜ wlt ⎝ 2 0
3 J ( β)expj wlt 2 1
(
)
+wrt+Φl( t−τ+) + Φr ( t−τ+) − ϕ2
⎛ 3 J ( β)expj⎜ wlt + 2wrt +Φl( t−τ−) ⎝ 2 2 π ⎞⎫ +2Φr ( t−τ−) − ϕ3 − ⎟⎬ 2 ⎠⎭ +
⎡ ⎤ A 0. L mod. ⎧ π ⎨ expj⎢ wlt+ + Φl( t)+βcos(wrt+Φr (t))⎥ ⎣ ⎦ ⎩ 2 2 ⎡ π ⎞⎤⎫ + expj⎢ wlt+Φl( t)+βcos(wrt+Φr ( t) + ⎟⎥⎬ ⎣ 2 ⎠⎦⎭
⎛ α fiberL ⎞ fiber ⎟⎧ −⎜ 20 ⎝ ⎠⎨
V120o( L, t)=A . L mod. L add. 10
signals are generated using 90o and 120o hybrid coupler respectively. Thus, the output of DDMZM based on 90o and 120o hybrid coupler can be expressed as:
V120o( 0, t) =
(8)
defines a nor-
phase shifted signal of x r(t), θ is the phase shift and β=
V90o( 0, t) =
)
)
malized dc, Vπ is the switching voltage of the DD-MZM, x̃ r(t) is the By setting the values of
r
⎛ π ⎞⎞ + 2.J2 ( β)expj⎜ wlt+Φl( t)+ 2w t + 2Φr( t) − ⎟⎟ r ⎝ 4 ⎠⎠
r
= Vdc Vπ
(
2 J1( β)expj wlt+Φl( t)+ w t+Φr( t)
+wrt+Φl( t−τ+) + Φr ( t−τ+) − ϕ2
r
where L mod is the insertion loss of DD-MZM, γ
⎛ π⎞ J0 ( β)expj⎜ wlt+Φl( t) + ⎟ − ⎝ 4⎠
+Φl( t−τ0) − ϕ1 +
{ expj⎡⎣ w t + γπ+Φ ( t)+βcos(w t+Φ (t))⎤⎦ 2 + expj⎡⎣ w t+Φ ( t)+βcos(w t+Φ ( t)+θ)⎤⎦} l
(7)
V 90 o( 0, t)
⎡ ⎡ x̃ t ⎤⎤ L mod. xl( t) ⎡ x t ⎤ ⎢ expj⎢ γπ+ π r( ) ⎥+expj⎢ π r( ) ⎥⎥ ⎢⎣ ⎢⎣ Vπ 2 ⎥⎦⎥⎦ Vπ 2 ⎥⎦ ⎢⎣ 2
A 0. L mod
⎡ ⎧ m =∞ ⎫ ⎪ A 0. L mod. ⎢ ⎪ π m ⎨ ∑ ( j) Jm ( β)expj(wlt+ + Φl(t) + mw t+mΦr( t))⎬ ⎪ ⎪ ⎢ 3 r 2 ⎭ ⎣ ⎩ m =−∞
The argument of Bessel function ,β,is very small as Vπ > Vr ,.So, the high order components can be negotiate. Thus, by considering the analysis up to second order terms and using the property of the Bessel function, J−n (β) = (−1)n Jn(β), the output signals are represented as:
(2)
where A 0 and Vr are amplitudes from the laser and the RF oscillator, wl and wr are the angular frequencies of the signals from the laser and the RF oscillator and Φl(t) and Φr(t) are phase noise. The output signal of the DD-MZM is expressed as:
(6)
m =∞ ⎤ ⎧ ⎫ ⎪ ⎪ ∑ ( j)mJm ( β)expj (wlt+Φl(t) + mwr t+mΦr( t)+m 2π )⎬⎪⎥⎥ +⎨ ⎪ 3 ⎩ m =−∞ ⎭⎦
+
x r( t) = Vr. cos(wrt+Φr (t))
V( 0, t) =
⎡ ⎧ m =∞ ⎫ ⎪ A 0. L mod. ⎢ ⎪ π m ⎨ ∑ ( j) Jm ( β)expj (wlt+ + Φl(t) + mw t+mΦr( t))⎬ ⎪ ⎢⎪ 2 r 2 ⎭ ⎣ ⎩ m =−∞
Hybrid Coupler
(11)
where Ladd defines an additional loss in the optical link, α fiber is the fiber loss and τ0, τ þ and τ− denotes group delays for a center angular frequency of wl, an upper sideband frequency of wl þ wr and wl þ2wr. ϕ1, ϕ2 and ϕ3 are phase shift parameters for specific frequencies due to the fiber chromatic dispersion. By using a square-law model, the photocurrent (PD), ip(t) is calculated as [11, 12]:
ip( t)≅ρ|V(L, t)|2 where
(12)
ρ denotes the responsivity of the PD and ∙ is the square2
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law detection.
given as:
⎡ ⎛ ip90 o( t)=ρA12⎢ B−2. α1cos⎜ wrt −Φl( t−τ0 )+Φ ( t−τ+) + Φr( t−τ+) + ϕ1 − ϕ 2 l ⎝ ⎣
τ1=D.L fiber. λ 20.
−
⎛ π ⎟⎞ +4.α 2cos⎜ 2wrt −Φl( t−τ0 )−Φl( t−τ−)+2Φr( t ⎝ 4⎠ ⎛ π⎞ −τ−) + ϕ1 − ϕ3 − ⎟ −4 2 C .cos⎜ wrt ⎝ 2⎠ + −Φl( t−τ+) + Φl( t−τ−)−Φr( t−τ+)+2Φr( t−τ−) + ϕ 2 − ϕ3 −
⎡ ip120 o( t)=ρA12⎢ B1 − ⎢⎣
π ⎞⎟⎤ ⎥ 4 ⎠⎦ (13)
α1 =
2 J1 ( β) J0 ( β)
, α2 =
J2 ( β) J0 ( β)
(14)
J0 ( β)
,B = 1+α12+4α 22,B1 =
+
τ1
∫τ
∞
1
2 2 J ( β)J ( β) 3 3 3 + α + α , C= 2 2 1 2 4 1 2 2 J0 ( β)
(15)
τ jwr −2γ)
∫0
τ1
∫τ
∞
1
(16)
where 2γr , 2γl define the angular full linewidth for the laser and the RF oscillator, respectively, 2γ is the total linewidth which is 9 2
1
1
−τ1
∫−∞
9
⎫ + eτ( −jwr −2γ) eτ( − jw)dτ⎬ ⎭
)
(e (
τ jwr +γr )
(e (
τ jwr −γr )
)
+ eτ( −jwr +γr) eτ( − jw)dτ
⎫ + eτ( −jwr −γr) eτ( − jw)dτ⎬ ⎭
)
(18)
2
given as γr +2γl and 2β12=2. α + 16. C2 and 2β22 = 4 α + 2 .C .
0
∫−τ ( eτ( jw +2γ) + eτ( −jw +2γ))eτ( − jw)dτ r
r
1
(e (
τ jwr −2γ)
⎧ + e−2γlτ1⎨ ⎩ +
2
r
ρ2. A14
+
⎧ B2 + 2β2cos( w τ)e−2γ τ , τ ≤ τ ⎫ ⎪ ⎪ 1 r 1 2 ⎬ =⎨ −2γlτ1 −γr τ 2 2 ⎪ e , τ > τ1⎪ ⎩ B1 + 2β2cos( wrτ)e ⎭
r
1
(e (
⎧ = B12δ( f) + β22⎨ ⎩
−2γ τ 2 2 ⎧ , τ ≤ τ1 ⎫ ⎪ RI90o( τ) ⎪ B + 2β1cos( wrτ)e ⎬ =⎨ 2 4 −2γlτ1 −γr τ 2 2 ⎪ ⎪ ρ . A1 + β ( τ) τ > τ B 2 cos w e e , r 1⎭ ⎩ 1
0
∫−τ ( eτ( jw +2γ) + eτ( −jw +2γ))eτ( − jw)dτ
SI120o ( f)
R I( τ) = ip( t). ip( t + τ)
ρ2. A14
∫0
⎧ + e−2γlτ1⎨ ⎩
The autocorrelation function RI(τ) is obtained as:
RI120o( τ)
SI ( f)=F RI(τ)
+
where
A1 = A 0. L mod. L add.
where c is the light velocity, fr defines the signal frequency, D denotes the fiber chromatic dispersion and λ 0 is the wavelength. The power spectral density function can be determined as:
⎧ = B2δ( f) + β12⎨ ⎩
)
⎛ α fiberL ⎞ fiber ⎟ −⎜ 20 ⎠. 10 ⎝
(17)
ρ2. A14
+ 3.α2. cos 2wrt −Φl( t−τ0)−Φl( t−τ−)+ 2Φr( t−τ−) + ϕ1 − ϕ 3 ⎛ − 3C1. cos⎜ wrt + −Φl( t−τ+) ⎝ π ⎞⎤ + Φl( t−τ−)−Φr( t−τ+)+ 2Φr( t−τ−) + ϕ 2 − ϕ 3 + ⎟⎥ 6 ⎠⎥⎦
fr c
SI90o ( f)
⎛ π⎞ .α cos⎜ wrt −Φl( t−τ0)+Φl( t−τ+) + Φr( t−τ+) + ϕ1 − ϕ 2 − ⎟ 6⎠ 2 1 ⎝
3
(
3
−τ1
∫−∞
⎫ + eτ( −jwr −2γ) eτ( − jw)dτ⎬ ⎭
)
(e (
(e (
τ jwr −γr )
τ jwr +γr )
)
+ eτ( −jwr +γr) eτ( − jw)dτ
⎫ + eτ( −jwr −γr) eτ( − jw)dτ⎬ ⎭
)
(19)
The received RF carrier power is obtained as:
PRFC=2
B fr + r 2 B fr − r 2
∫
The differential delay due to the fiber chromatic dispersion is
PRFC90o =
SI ( f)df
4ρ2A14β12 π
⎛ πB ⎞ e−2γτ1. tan−1⎜⎜ r ⎟⎟ ⎝ γr ⎠
(20)
77 76
PRFC120o =
4ρ2A14β22 π
⎛ πB ⎞ e−2γτ1. tan−1⎜⎜ r ⎟⎟ ⎝ γr ⎠
(21)
Signal to Noise ratio in dB
75 74
For evaluating the total RF power excluding dc power, utilizing Eqs. (15), (16) and putting τ1=0. Total Power is obtained as:
73
PT90o=2ρ2A14β12
(22)
PT120o=2ρ2A14β22
(23)
72 71
The percentage of received power which considers the effect of the filter bandwidth (Br ) at an electrical receiver,is found as the ratio between the total carrier power and the required power as:
OSSB RoF System Based on 120 degree Hybrid Coupler OSSB RoF System Based on 90 degree Hybrid Coupler
70 69 0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
⎛ π .B ⎞ 2 r⎟ p= e−2γτ1. tan−1⎜⎜ ⎟ π ⎝ γr ⎠
(24)
Percentage of Received Power Fig. 1.2. SNR variation as a function of percentage of received power for 90o and 120o Hybrid Coupler OSSB RoF system.
The performance measuring parameter, SNR, is evaluated as the ratio signal power to noise power as:
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4
⎛ ⎞ P SNR = 10log10⎜ N RFC ⎟ ⎜ 0 .B ⎟ ⎝ 2 r⎠
95
90
(
)
⎛ 2ρ2A14β22.p SNR120o=10log10⎜⎜ γr π 2γτ1 ⎝ N0 . π . tan 2 .p.e
(
⎞ ⎟ ⎟ ⎠
)
⎞ ⎟ ⎟ ⎠
OSSB RoF System Based on 90 degree Hybrid Coupler OSSB RoF System Based on 120 degree Hybrid Coupler
(25)
(26)
Thus, it is observed that the SNR is dependent on the percentage of received power which is function of required bandwidth, phase noise from RF oscillator & laser linewidth, length of fiber and RF input signal power.
Signal to Noise ratio in dB
⎛ 2ρ2A14β12.p SNR 90o=10log10⎜⎜ γr π 2γτ1 ⎝ N0 . π . tan 2 .p.e
85
80
75
70
65 0
1
2
3
3. Results and discussion
5
6
7
8
9
10
Fig. 1.3. SNR variation as a function of RF oscillator linewidth for 90o and 120o Hybrid Coupler OSSB RoF system.
89 88
Signal to Noise ratio in dB
87 86 85 84 83 82 81 OSSB RoF System Based on 120 degree Hybrid Coupler OSSB RoF System Based on 90 degree Hybrid Coupler
80 79 0
1000
2000
3000
4000
5000
6000
7000
8000
Length of Optical Fiber in meter Fig. 1.4. SNR variation as a function of optical fiber length for 90o and 120o Hybrid Coupler OSSB RoF system.
88 87.9 87.8 Signal to Noise ratio in dB
The presented analysis has been utilized to carry out numerical simulations for signal to noise ratio (SNR) of RoF transmission system. Figs 1.2–1.6 show the variation of SNR as a function of various parameters for 90o and 120o Hybrid Coupler OSSB RoF system that influence the performance significantly. The results have been reported by taking values of parameters such as wavelength of laser diode¼ 1550 nm, power of laser diode¼ 0 dBm, switching voltage of DD-MZM¼2.5 V, responsivity¼ 0.6, DD-MZM insertion loss¼3 dB, optical fiber loss¼ 0.2 dB/km, dispersion parameter¼17 ps/km.nm and RF carrier frequency¼ 30 GHz. In the Fig. 1.2, SNR is observed for different values of percentage of received power which is varied from 0.1 to 0.9. The value for laser linewidth is kept at 624 MHz, RF oscillator linewidth at 1 Hz, optical fiber length at 8 km and RF signal input power at 20 dBm. It can be seen that at a percentage of received power of 0.1 value, SNR peak value is 76 dB and 76.52 dB for 90 o and 120 o Hybrid Coupler OSSB RoF system. An increment of 0.52 dB is observed in the peak value of SNR while adopting 120o Hybrid Coupler in comparison with 90o Hybrid Coupler. Hence, the system does work efficiently at 120o Hybrid Coupler. SNR decreases as p becomes large since the increment of the noise power is greater than that of the received signal power as the bandwidth increases. Thus, the bandwidth should be considered carefully for p > 0. 9, since the SNR decreases drastically over the point as a result. For example, the SNR at p = 0. 9 is 70 dB and SNR is 76.52 dB at p = 0. 1. Thus, the minimum required power to detect the signal should be carefully considered before we choose the filter bandwidth. A 6.52 dB reduction in SNR peak values is observed with increase in percentage of received power. Fig. 1.3 shows SNR variation for varied RF oscillator linewidth values. The RF oscillator linewidth has been swept from 0.1 to 10 Hz. The value for laser linewidth is kept at 624 MHz, percentage of received power at 0.5, optical fiber length at 8 km and RF signal input power at 20 dBm. It can be seen that SNR decreases with increases in RF oscillator linewidth. For 90o hybrid coupler based system, SNR has a value of 85.30 and 65 dB at RF oscillator linewidth of 0.1 and 10 Hz respectively. Similarly, for 120o hybrid coupler based system, SNR values are observed as 90.94 and 70.64 for 0.1 and 10 Hz, RF oscillator linewidth respectively. Thus, an increment of 5.64 dB is observed in the peak value of SNR while adopting 120o Hybrid Coupler in comparison with 90o Hybrid Coupler. The influence of phase noise of RF signal is quite prominent on system performance. Fig. 1.4 shows the variation of SNR as a function of optical fiber
4
RF Oscillator linewidth in Hz
87.7 OSSB RoF System Based on 120 degree Hybrid Coupler OSSB RoF System Based on 90 degree Hybrid Coupler
87.6 87.5 87.4 87.3 87.2 0
100
200
300
400
500
600
700
Optical Source Laser Linewidth in MHz
Fig. 1.5. SNR variation as a function of Laser linewidth for 90o and 120o Hybrid Coupler OSSB RoF system.
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value of SNR while adopting 120o Hybrid Coupler in comparison with 90o Hybrid Coupler. Thus, it can be stated that the phase noise from RF oscillator influences the SNR significantly. The SNR degradation due to RF oscillator phase noise is much more than that from the laser linewidth.
76 OSSB RoF System Based on 120 degree Hybrid Coupler OSSB RoF System Based on 90 degree Hybrid Coupler
75 74 Signal to Noise ratio in dB
5
73 72 71
4. Conclusion
70
Analytical models for OSSB RoF system based on a DD-MZM using 90o and 120o hybrid coupler have been developed. Analysis and simulation have been carried out to understand & compare the performance of two systems. An improvement in the performance in terms of SNR has been found considering degradation parameters such as phase noise from RF signal oscillator and laser, input RF signal power and fiber dispersion for the system. An improvement of 5.64 and 0.67 dB has been observed when RF and laser phase noise increase in 120o hybrid coupler based system over conventional 90o hybrid coupler system. Thus, OSSB RoF 120o Hybrid Coupler system works efficiently providing good performance.
69 68 67 66 0
2
4
6
8
10
12
14
16
18
20
Input RF Signal Power in dBm Fig. 1.6. SNR variation as a function of input RF signal power for 90o and 120o Hybrid Coupler OSSB RoF system.
length. Keeping the range of microcell in view, the fiber length is varied from 0 to 8 Km. The value for laser linewidth is kept at 624 MHz, RF oscillator linewidth at 1 Hz, percentage of received power at 0.5 and RF signal input power at 20 dBm. It is found that as the optical fiber length increases, SNR decreases. SNR decreases from 88.5 to 80.3 dB as fiber length increases. A decrement of 8.2 dB is observed in the SNR with increase in optical fiber length. This is due to the fact that various factors including dispersion deteriorate the output of the system. But, an improvement of 0.74 dB is observed based on 120o Hybrid Coupler based system design. Fig. 1.5 shows the effect of Laser linewidth, which is varied from 0 to 700 MHz, on SNR. The value for optical fiber length is kept at 8 km, RF oscillator linewidth at 1 Hz, percentage of received power at 0.5 and RF signal input power at 20 dBm. Laser linewidth does not seem to be the dominant degradation factor as the change is very small i.e less than 0.01 dB. The laser linewidth has been chosen keeping in view the linewidth of commercially available Distributed feedback laser (DFB) and febryperot (FP) laser i.e. 10 MHz & 624 MHz. Transmission distance is selected in accordance with the range of radius of microcell which is 1 to 10 Km. An increment of 0.67 dB is observed in the peak value of SNR while adopting 120o Hybrid Coupler in comparison with 90o Hybrid Coupler. The effect RF oscillator is more prominent. This means that we can employ a cheap laser such as the FP laser in the RoF system in microcell without a severe SNR degradation. Fig. 1.6 shows the variation of SNR as a function of input RF signal power. The value for optical fiber length is kept at 8 km, RF oscillator linewidth at 1 Hz, percentage of received power at 0.5 and laser linewidth at 624 MHz. It can be seen that SNR increases as an increment in input RF signal power. SNR increases by almost 8.6 dB with increase in input RF signal power from 0 to 20 dB. Further, an increment of 0.5339 dB is observed in the peak
References [1] Z.Z. Cao, J.J. Yu, L. Chen, Q.L. Shu, Reversely modulated optical single sideband scheme and its application in a 60-GHz full duplex RoF system, IEEE Photon. Technol. Letter vol. 24 (no. 10) (2012) 827–829 , May. [2] T.G. Ning, J. Li, L. Pei, F. Zhang, Q.A. Zhou, X.D. Wen, C.H. Qi, J.J. Zheng, Overwritten fiber Bragg grating and its application in an optical single sideband with carrier modulation radio over a fiber system, Optical Engineering vol. 50 (2011) 035001-1–035001-5 , Mar. [3] H.J. Zhou, W. Chen, Z. Meng, Optical single sideband-frequency generation with carrier totally suppressed for Brillouin distributed fiber sensing, Opt. Commun. vol. 285 (2012) 4391–4394. [4] J.E. Roman, M.Y. Frankel, R.D. Esman, Spectral characterization of fiber gratings with high resolution, Opt. Lett. vol. 23 (1998) 939–941. [5] Z.Z. Tang, S.L. Pan, J.P. Yao, A high resolution optical vector network analyzer based on a wideband and wavelength-tunable optical single-sideband modulator, Opt. Exp vol. 20 (2012) 6555–6560. [6] S.L. Pan and M. Xue, “Optical vector network analyzer based on optical singlesideband modulation,” presented at the 12th Int. Conf. Optical Communications and Networks, Chengdu, China, 2013. [7] H. Mima and K. Iwashita, “A novel wavelength converter based on optical single-sideband modulator and arrayed waveguide grating,” in Proc. OptoElectron. Commun. Conf., pp. 327–328, 2012. [8] T. Yamamoto, R. Kohno, A. Chiba, T. Sakamoto, and T. Kawanishi, “Agile wideband light source for optical coherence tomography using optical SSB frequency sweep technique,” in Proc. Dig. IEEE/LEOS Summer Top. Meet, pp. 137–138, 2007. [9] J. Sun n, L. Yu, Y. Zhong, A single sideband radio-over-fiber system with improved dynamic range incorporating a dual-electrode dual-parallel Mach– Zehnder modulator, Optics Communications (2015). [10] Z. Li, H. Chi, X.M. Zhang, J.P. Yao, Optical single-sideband modulation using a fiber-Bragg-grating-based optical Hilbert transformer, IEEE Photon. Technol. Lett vol. 23 (no. 9) (2011) 558–560. [11] V. Sharma, A. Singh, A.K. Sharma, Simulative investigation on the impact of laser-spectral width in single-tone radio-over-fiber transmission system using optical single side-band technique, Optics and Lasers in Engineering (2009). [12] S. Singla, S.K. Arya, Impact of higher order dispersion on phase induced power penalty for RoF systems, International Journal for Light and Electron Optics, Optik vol. 24 (no.14) (2013) 1917–1920. [13] M. Xue, S. Pan, Y. Zhao, Optical Single-Sideband Modulation Based on a DualDrive MZM and a 120° Hybrid Coupler, Journal of Lightwave Technology vol. 32 (no. 19) (2014).
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Performance analysis of an OSSB RoF link using 90o & 120o Hybrid coupler Parvin Kumar a, Sanjay Kumar Sharma a, Shelly Singla b a b
Krishna Institute of Engineering and Technology, Ghaziabad (U.P.)-201206, India Indus Institute of Engineering and Technology, Kinana (Haryana)-126114, India
art ic l e i nf o
a b s t r a c t
Article history: Received 23 July 2015 Received in revised form 14 September 2015 Accepted 22 September 2015
This paper presents an analysis, simulation and comparison of the performance of Optical single sideband (OSSB) radio over fiber (RoF) system based on a dual drive Mach Zehender modulator (DD-MZM) using 90o and 120o hybrid coupler including the effects of phase noise from RF signal oscillator and laser source, input RF signal power and fiber dispersion. Signal to Noise ratio (SNR) is significantly influenced by phase noise from RF signal oscillator and input RF signal power. On comparison with conventional 90o hybrid coupler system, the performance of the considered system improves by 5.64 and 0.67 dB in terms of SNR, when RF and laser phase noises are increased. & 2015 Elsevier B.V. All rights reserved.
Keywords: Optical single sideband (OSSB) Radio over Fiber (RoF) Dual drive Mach Zehender modulator (DDMZM) phase noise from a laser and an RF oscillator and Signal to Noise ratio (SNR)
1. Introduction A Radio over Fiber (RoF) system is a promising technique for next generation wireless networks as it has capability to achieve multi-gigabit per second data rate to support bandwidth intensive applications. Optical single sideband (OSSB) modulation has become an attractive technology for achieving long distance RoF based transmission highly accurate optical sensing [1–3], highresolution optical vector network analysis [4–6], optical wavelength conversion [7], and optical coherence tomography [8]. The OSSB signal is not in affected by the errors induced by fiber dispersion, which is severe in the conventional double sideband (DSB) modulated RoF transmission system. OSSB modulation can be implemented using various schemes [9,10]. Majority two schemes are preferred optical filtering and 90° phase shift procedure. In the optical filtering method, the OSSB signal is generated by removing one of the first order sidebands in an intensity modulated or phase modulated signal. While the OSSB modulation signal generated using a 90° phase shift method is almost independent of the wavelength of the optical carrier. In this method, two RF signals with a phase difference of 90° are fed to a dual drive Mach Zehender modulator (DDMZM) [11,12]. The signal in the upper and lower arms of the MZM have a phase difference of nπ/2 −∅0 , where nrepresents the order E-mail addresses: [email protected] (P. Kumar), [email protected] (S. Kumar Sharma), [email protected] (S. Singla).
of the sideband and ∅0 is the phase difference introduced by the dc bias. When ∅0=−π/2 or π/2, one of the first order sidebands would be cancelled out when the signals in the upper and lower arms are combined at the output port of the modulator. Thus, OSSB modulated signal is generated. When ∅0=−π/2, the 1st and þ3rd order sidebands are effectively suppressed. But the unwanted þ2nd and higher order sidebands still exist. The existence of the þ2nd order sideband causes power degradation due to the fiber dispersion. To overcome these problems it is desirable to eliminate 2nd order sideband. Keeping this in view, a novel OSSB modulation based on a 120° hybrid coupler and a DD-MZM is proposed and demonstrated by [13]. A RoF system suffers from various performance degradation parameters like phase noise, input RF signal power and high frequency band. It is highly desirable to evaluate performance of the system including all these errors. Thus, in this paper analysis has been done against these parameters for 90° & 120° hybrid coupler based systems. Simulations have also been carried out to evaluate the performance and draw comparison between them.
2. System Model Fig. 1.1 shows the implementation of OSSB RoF system based on a DD-MZM. With a proper dc bias, the phase difference between optical signals in the upper and lower arms of the DD-MZM is controlled. In a 90o phase shift method, the phase difference between the optical components is nπ/2- ∅0 while for 120o hybrid
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Please cite this article as: P. Kumar, et al., Optics Communications (2015), http://dx.doi.org/10.1016/j.optcom.2015.09.081i
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2
RF oscillator
V 90 o( 0, t) =
Optical Fiber Laser Diode
Electrical Spectrum Analyzer
Photodetector
DD-MZM DC Bias ( ) Optical Path
m =∞ ⎤ ⎧ ⎫ ⎪ ⎪ +⎨ ∑ ( j)mJm ( β)expj(wlt+Φl(t) + mwr t+mΦr( t)+m π )⎬⎪⎥⎥ ⎪ 2 ⎭⎦ ⎩ m =−∞
Electrical Path
Fig. 1.1. Block diagram of OSSB RoF system based on a dual drive Mach Zehender modulator.
coupler, the phase difference is nπ/3-∅0 . ∅0=π /3, the -1 and þ2 order sideband are simultaneously suppressed because these sidebands have a -180o and 180o phase difference respectively and are destructively inferred when combined at the output port of the DD-MZM. As a result, an OSSB signal with both of the -1st order sideband and þ2nd order sideband suppressed is generated. Similarly, when ∅0= − π /3,an OSSB signal with both of the þ 1st order sideband and -2nd order sideband suppressed is generated [13]. The signal from the laser diode and the RF oscillator are modeled as: st
nd
xl( t) = A 0expj( wlt+Φl(t))
(1)
V120 o( 0, t) =
V( 0, t) =
⎛ = A 0. L mod. ⎜ ⎝
l
l
l
r
r
V120 o( 0, t) ⎛ = A 0. L mod. ⎜ ⎝
⎛ 3 π⎞ J ( β)expj⎜ wlt+Φl( t) + ⎟ − ⎝ 2 0 6⎠
3 J ( β)expj wlt+Φl( t)+ w t+Φr( t) r 2 1
(
)
⎛ 3 π ⎞⎞ J ( β)expj⎜ wlt+Φl( t)+ 2w t + 2Φr( t) − ⎟⎟ r ⎝ 2 2 2 ⎠⎠
(9)
After the transmission over distance of L fiber km on optical fiber, the signals are modeled as: ⎛ α fiberL ⎞ fiber ⎟ −⎜ ⎧ 20 ⎝ ⎠⎨ J ⎩ 0
V90o( L, t)=A . L mod. L add. 10 0
π ⎟⎞ − 4⎠
⎛
( β). expj⎝ wlt ⎜
(
2 J1( β).expj wlt
⎛ +2.J2 ( β). expj⎜ wlt + 2wrt ⎝ +Φl( t−τ−)+2Φr ( t−τ−) − ϕ3 −
(3)
πVr 2 Vπ
.
0
θ and γ as ( 2π , 21 ) and (2 3π , 31 ), the OSSB
+Φl( t−τ0) − ϕ1 +
π ⎟⎞ 4⎠
(10)
(4)
⎡ ⎤ A 0. L mod. ⎧ π ⎨ expj⎢ wlt+ + Φl( t)+βcos(wrt+Φr (t))⎥ ⎣ ⎦ ⎩ 3 2 ⎡ 2π ⎤⎫ + expj⎢ wlt+Φl( t)+βcos(wrt+Φr ( t) + )⎥⎬ ⎣ 3 ⎦⎭
(5)
Using the above equations and defined functions, the output of DD-MZM can be represented in terms of Bessel function as:
⎩
π ⎞⎟ − 6⎠
⎛ 3 J ( β). expj⎜ wlt ⎝ 2 0
3 J ( β)expj wlt 2 1
(
)
+wrt+Φl( t−τ+) + Φr ( t−τ+) − ϕ2
⎛ 3 J ( β)expj⎜ wlt + 2wrt +Φl( t−τ−) ⎝ 2 2 π ⎞⎫ +2Φr ( t−τ−) − ϕ3 − ⎟⎬ 2 ⎠⎭ +
⎡ ⎤ A 0. L mod. ⎧ π ⎨ expj⎢ wlt+ + Φl( t)+βcos(wrt+Φr (t))⎥ ⎣ ⎦ ⎩ 2 2 ⎡ π ⎞⎤⎫ + expj⎢ wlt+Φl( t)+βcos(wrt+Φr ( t) + ⎟⎥⎬ ⎣ 2 ⎠⎦⎭
⎛ α fiberL ⎞ fiber ⎟⎧ −⎜ 20 ⎝ ⎠⎨
V120o( L, t)=A . L mod. L add. 10
signals are generated using 90o and 120o hybrid coupler respectively. Thus, the output of DDMZM based on 90o and 120o hybrid coupler can be expressed as:
V120o( 0, t) =
(8)
defines a nor-
phase shifted signal of x r(t), θ is the phase shift and β=
V90o( 0, t) =
)
)
malized dc, Vπ is the switching voltage of the DD-MZM, x̃ r(t) is the By setting the values of
r
⎛ π ⎞⎞ + 2.J2 ( β)expj⎜ wlt+Φl( t)+ 2w t + 2Φr( t) − ⎟⎟ r ⎝ 4 ⎠⎠
r
= Vdc Vπ
(
2 J1( β)expj wlt+Φl( t)+ w t+Φr( t)
+wrt+Φl( t−τ+) + Φr ( t−τ+) − ϕ2
r
where L mod is the insertion loss of DD-MZM, γ
⎛ π⎞ J0 ( β)expj⎜ wlt+Φl( t) + ⎟ − ⎝ 4⎠
+Φl( t−τ0) − ϕ1 +
{ expj⎡⎣ w t + γπ+Φ ( t)+βcos(w t+Φ (t))⎤⎦ 2 + expj⎡⎣ w t+Φ ( t)+βcos(w t+Φ ( t)+θ)⎤⎦} l
(7)
V 90 o( 0, t)
⎡ ⎡ x̃ t ⎤⎤ L mod. xl( t) ⎡ x t ⎤ ⎢ expj⎢ γπ+ π r( ) ⎥+expj⎢ π r( ) ⎥⎥ ⎢⎣ ⎢⎣ Vπ 2 ⎥⎦⎥⎦ Vπ 2 ⎥⎦ ⎢⎣ 2
A 0. L mod
⎡ ⎧ m =∞ ⎫ ⎪ A 0. L mod. ⎢ ⎪ π m ⎨ ∑ ( j) Jm ( β)expj(wlt+ + Φl(t) + mw t+mΦr( t))⎬ ⎪ ⎪ ⎢ 3 r 2 ⎭ ⎣ ⎩ m =−∞
The argument of Bessel function ,β,is very small as Vπ > Vr ,.So, the high order components can be negotiate. Thus, by considering the analysis up to second order terms and using the property of the Bessel function, J−n (β) = (−1)n Jn(β), the output signals are represented as:
(2)
where A 0 and Vr are amplitudes from the laser and the RF oscillator, wl and wr are the angular frequencies of the signals from the laser and the RF oscillator and Φl(t) and Φr(t) are phase noise. The output signal of the DD-MZM is expressed as:
(6)
m =∞ ⎤ ⎧ ⎫ ⎪ ⎪ ∑ ( j)mJm ( β)expj (wlt+Φl(t) + mwr t+mΦr( t)+m 2π )⎬⎪⎥⎥ +⎨ ⎪ 3 ⎩ m =−∞ ⎭⎦
+
x r( t) = Vr. cos(wrt+Φr (t))
V( 0, t) =
⎡ ⎧ m =∞ ⎫ ⎪ A 0. L mod. ⎢ ⎪ π m ⎨ ∑ ( j) Jm ( β)expj (wlt+ + Φl(t) + mw t+mΦr( t))⎬ ⎪ ⎢⎪ 2 r 2 ⎭ ⎣ ⎩ m =−∞
Hybrid Coupler
(11)
where Ladd defines an additional loss in the optical link, α fiber is the fiber loss and τ0, τ þ and τ− denotes group delays for a center angular frequency of wl, an upper sideband frequency of wl þ wr and wl þ2wr. ϕ1, ϕ2 and ϕ3 are phase shift parameters for specific frequencies due to the fiber chromatic dispersion. By using a square-law model, the photocurrent (PD), ip(t) is calculated as [11, 12]:
ip( t)≅ρ|V(L, t)|2 where
(12)
ρ denotes the responsivity of the PD and ∙ is the square2
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law detection.
given as:
⎡ ⎛ ip90 o( t)=ρA12⎢ B−2. α1cos⎜ wrt −Φl( t−τ0 )+Φ ( t−τ+) + Φr( t−τ+) + ϕ1 − ϕ 2 l ⎝ ⎣
τ1=D.L fiber. λ 20.
−
⎛ π ⎟⎞ +4.α 2cos⎜ 2wrt −Φl( t−τ0 )−Φl( t−τ−)+2Φr( t ⎝ 4⎠ ⎛ π⎞ −τ−) + ϕ1 − ϕ3 − ⎟ −4 2 C .cos⎜ wrt ⎝ 2⎠ + −Φl( t−τ+) + Φl( t−τ−)−Φr( t−τ+)+2Φr( t−τ−) + ϕ 2 − ϕ3 −
⎡ ip120 o( t)=ρA12⎢ B1 − ⎢⎣
π ⎞⎟⎤ ⎥ 4 ⎠⎦ (13)
α1 =
2 J1 ( β) J0 ( β)
, α2 =
J2 ( β) J0 ( β)
(14)
J0 ( β)
,B = 1+α12+4α 22,B1 =
+
τ1
∫τ
∞
1
2 2 J ( β)J ( β) 3 3 3 + α + α , C= 2 2 1 2 4 1 2 2 J0 ( β)
(15)
τ jwr −2γ)
∫0
τ1
∫τ
∞
1
(16)
where 2γr , 2γl define the angular full linewidth for the laser and the RF oscillator, respectively, 2γ is the total linewidth which is 9 2
1
1
−τ1
∫−∞
9
⎫ + eτ( −jwr −2γ) eτ( − jw)dτ⎬ ⎭
)
(e (
τ jwr +γr )
(e (
τ jwr −γr )
)
+ eτ( −jwr +γr) eτ( − jw)dτ
⎫ + eτ( −jwr −γr) eτ( − jw)dτ⎬ ⎭
)
(18)
2
given as γr +2γl and 2β12=2. α + 16. C2 and 2β22 = 4 α + 2 .C .
0
∫−τ ( eτ( jw +2γ) + eτ( −jw +2γ))eτ( − jw)dτ r
r
1
(e (
τ jwr −2γ)
⎧ + e−2γlτ1⎨ ⎩ +
2
r
ρ2. A14
+
⎧ B2 + 2β2cos( w τ)e−2γ τ , τ ≤ τ ⎫ ⎪ ⎪ 1 r 1 2 ⎬ =⎨ −2γlτ1 −γr τ 2 2 ⎪ e , τ > τ1⎪ ⎩ B1 + 2β2cos( wrτ)e ⎭
r
1
(e (
⎧ = B12δ( f) + β22⎨ ⎩
−2γ τ 2 2 ⎧ , τ ≤ τ1 ⎫ ⎪ RI90o( τ) ⎪ B + 2β1cos( wrτ)e ⎬ =⎨ 2 4 −2γlτ1 −γr τ 2 2 ⎪ ⎪ ρ . A1 + β ( τ) τ > τ B 2 cos w e e , r 1⎭ ⎩ 1
0
∫−τ ( eτ( jw +2γ) + eτ( −jw +2γ))eτ( − jw)dτ
SI120o ( f)
R I( τ) = ip( t). ip( t + τ)
ρ2. A14
∫0
⎧ + e−2γlτ1⎨ ⎩
The autocorrelation function RI(τ) is obtained as:
RI120o( τ)
SI ( f)=F RI(τ)
+
where
A1 = A 0. L mod. L add.
where c is the light velocity, fr defines the signal frequency, D denotes the fiber chromatic dispersion and λ 0 is the wavelength. The power spectral density function can be determined as:
⎧ = B2δ( f) + β12⎨ ⎩
)
⎛ α fiberL ⎞ fiber ⎟ −⎜ 20 ⎠. 10 ⎝
(17)
ρ2. A14
+ 3.α2. cos 2wrt −Φl( t−τ0)−Φl( t−τ−)+ 2Φr( t−τ−) + ϕ1 − ϕ 3 ⎛ − 3C1. cos⎜ wrt + −Φl( t−τ+) ⎝ π ⎞⎤ + Φl( t−τ−)−Φr( t−τ+)+ 2Φr( t−τ−) + ϕ 2 − ϕ 3 + ⎟⎥ 6 ⎠⎥⎦
fr c
SI90o ( f)
⎛ π⎞ .α cos⎜ wrt −Φl( t−τ0)+Φl( t−τ+) + Φr( t−τ+) + ϕ1 − ϕ 2 − ⎟ 6⎠ 2 1 ⎝
3
(
3
−τ1
∫−∞
⎫ + eτ( −jwr −2γ) eτ( − jw)dτ⎬ ⎭
)
(e (
(e (
τ jwr −γr )
τ jwr +γr )
)
+ eτ( −jwr +γr) eτ( − jw)dτ
⎫ + eτ( −jwr −γr) eτ( − jw)dτ⎬ ⎭
)
(19)
The received RF carrier power is obtained as:
PRFC=2
B fr + r 2 B fr − r 2
∫
The differential delay due to the fiber chromatic dispersion is
PRFC90o =
SI ( f)df
4ρ2A14β12 π
⎛ πB ⎞ e−2γτ1. tan−1⎜⎜ r ⎟⎟ ⎝ γr ⎠
(20)
77 76
PRFC120o =
4ρ2A14β22 π
⎛ πB ⎞ e−2γτ1. tan−1⎜⎜ r ⎟⎟ ⎝ γr ⎠
(21)
Signal to Noise ratio in dB
75 74
For evaluating the total RF power excluding dc power, utilizing Eqs. (15), (16) and putting τ1=0. Total Power is obtained as:
73
PT90o=2ρ2A14β12
(22)
PT120o=2ρ2A14β22
(23)
72 71
The percentage of received power which considers the effect of the filter bandwidth (Br ) at an electrical receiver,is found as the ratio between the total carrier power and the required power as:
OSSB RoF System Based on 120 degree Hybrid Coupler OSSB RoF System Based on 90 degree Hybrid Coupler
70 69 0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
⎛ π .B ⎞ 2 r⎟ p= e−2γτ1. tan−1⎜⎜ ⎟ π ⎝ γr ⎠
(24)
Percentage of Received Power Fig. 1.2. SNR variation as a function of percentage of received power for 90o and 120o Hybrid Coupler OSSB RoF system.
The performance measuring parameter, SNR, is evaluated as the ratio signal power to noise power as:
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4
⎛ ⎞ P SNR = 10log10⎜ N RFC ⎟ ⎜ 0 .B ⎟ ⎝ 2 r⎠
95
90
(
)
⎛ 2ρ2A14β22.p SNR120o=10log10⎜⎜ γr π 2γτ1 ⎝ N0 . π . tan 2 .p.e
(
⎞ ⎟ ⎟ ⎠
)
⎞ ⎟ ⎟ ⎠
OSSB RoF System Based on 90 degree Hybrid Coupler OSSB RoF System Based on 120 degree Hybrid Coupler
(25)
(26)
Thus, it is observed that the SNR is dependent on the percentage of received power which is function of required bandwidth, phase noise from RF oscillator & laser linewidth, length of fiber and RF input signal power.
Signal to Noise ratio in dB
⎛ 2ρ2A14β12.p SNR 90o=10log10⎜⎜ γr π 2γτ1 ⎝ N0 . π . tan 2 .p.e
85
80
75
70
65 0
1
2
3
3. Results and discussion
5
6
7
8
9
10
Fig. 1.3. SNR variation as a function of RF oscillator linewidth for 90o and 120o Hybrid Coupler OSSB RoF system.
89 88
Signal to Noise ratio in dB
87 86 85 84 83 82 81 OSSB RoF System Based on 120 degree Hybrid Coupler OSSB RoF System Based on 90 degree Hybrid Coupler
80 79 0
1000
2000
3000
4000
5000
6000
7000
8000
Length of Optical Fiber in meter Fig. 1.4. SNR variation as a function of optical fiber length for 90o and 120o Hybrid Coupler OSSB RoF system.
88 87.9 87.8 Signal to Noise ratio in dB
The presented analysis has been utilized to carry out numerical simulations for signal to noise ratio (SNR) of RoF transmission system. Figs 1.2–1.6 show the variation of SNR as a function of various parameters for 90o and 120o Hybrid Coupler OSSB RoF system that influence the performance significantly. The results have been reported by taking values of parameters such as wavelength of laser diode¼ 1550 nm, power of laser diode¼ 0 dBm, switching voltage of DD-MZM¼2.5 V, responsivity¼ 0.6, DD-MZM insertion loss¼3 dB, optical fiber loss¼ 0.2 dB/km, dispersion parameter¼17 ps/km.nm and RF carrier frequency¼ 30 GHz. In the Fig. 1.2, SNR is observed for different values of percentage of received power which is varied from 0.1 to 0.9. The value for laser linewidth is kept at 624 MHz, RF oscillator linewidth at 1 Hz, optical fiber length at 8 km and RF signal input power at 20 dBm. It can be seen that at a percentage of received power of 0.1 value, SNR peak value is 76 dB and 76.52 dB for 90 o and 120 o Hybrid Coupler OSSB RoF system. An increment of 0.52 dB is observed in the peak value of SNR while adopting 120o Hybrid Coupler in comparison with 90o Hybrid Coupler. Hence, the system does work efficiently at 120o Hybrid Coupler. SNR decreases as p becomes large since the increment of the noise power is greater than that of the received signal power as the bandwidth increases. Thus, the bandwidth should be considered carefully for p > 0. 9, since the SNR decreases drastically over the point as a result. For example, the SNR at p = 0. 9 is 70 dB and SNR is 76.52 dB at p = 0. 1. Thus, the minimum required power to detect the signal should be carefully considered before we choose the filter bandwidth. A 6.52 dB reduction in SNR peak values is observed with increase in percentage of received power. Fig. 1.3 shows SNR variation for varied RF oscillator linewidth values. The RF oscillator linewidth has been swept from 0.1 to 10 Hz. The value for laser linewidth is kept at 624 MHz, percentage of received power at 0.5, optical fiber length at 8 km and RF signal input power at 20 dBm. It can be seen that SNR decreases with increases in RF oscillator linewidth. For 90o hybrid coupler based system, SNR has a value of 85.30 and 65 dB at RF oscillator linewidth of 0.1 and 10 Hz respectively. Similarly, for 120o hybrid coupler based system, SNR values are observed as 90.94 and 70.64 for 0.1 and 10 Hz, RF oscillator linewidth respectively. Thus, an increment of 5.64 dB is observed in the peak value of SNR while adopting 120o Hybrid Coupler in comparison with 90o Hybrid Coupler. The influence of phase noise of RF signal is quite prominent on system performance. Fig. 1.4 shows the variation of SNR as a function of optical fiber
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RF Oscillator linewidth in Hz
87.7 OSSB RoF System Based on 120 degree Hybrid Coupler OSSB RoF System Based on 90 degree Hybrid Coupler
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Fig. 1.5. SNR variation as a function of Laser linewidth for 90o and 120o Hybrid Coupler OSSB RoF system.
Please cite this article as: P. Kumar, et al., Optics Communications (2015), http://dx.doi.org/10.1016/j.optcom.2015.09.081i
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value of SNR while adopting 120o Hybrid Coupler in comparison with 90o Hybrid Coupler. Thus, it can be stated that the phase noise from RF oscillator influences the SNR significantly. The SNR degradation due to RF oscillator phase noise is much more than that from the laser linewidth.
76 OSSB RoF System Based on 120 degree Hybrid Coupler OSSB RoF System Based on 90 degree Hybrid Coupler
75 74 Signal to Noise ratio in dB
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73 72 71
4. Conclusion
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Analytical models for OSSB RoF system based on a DD-MZM using 90o and 120o hybrid coupler have been developed. Analysis and simulation have been carried out to understand & compare the performance of two systems. An improvement in the performance in terms of SNR has been found considering degradation parameters such as phase noise from RF signal oscillator and laser, input RF signal power and fiber dispersion for the system. An improvement of 5.64 and 0.67 dB has been observed when RF and laser phase noise increase in 120o hybrid coupler based system over conventional 90o hybrid coupler system. Thus, OSSB RoF 120o Hybrid Coupler system works efficiently providing good performance.
69 68 67 66 0
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Input RF Signal Power in dBm Fig. 1.6. SNR variation as a function of input RF signal power for 90o and 120o Hybrid Coupler OSSB RoF system.
length. Keeping the range of microcell in view, the fiber length is varied from 0 to 8 Km. The value for laser linewidth is kept at 624 MHz, RF oscillator linewidth at 1 Hz, percentage of received power at 0.5 and RF signal input power at 20 dBm. It is found that as the optical fiber length increases, SNR decreases. SNR decreases from 88.5 to 80.3 dB as fiber length increases. A decrement of 8.2 dB is observed in the SNR with increase in optical fiber length. This is due to the fact that various factors including dispersion deteriorate the output of the system. But, an improvement of 0.74 dB is observed based on 120o Hybrid Coupler based system design. Fig. 1.5 shows the effect of Laser linewidth, which is varied from 0 to 700 MHz, on SNR. The value for optical fiber length is kept at 8 km, RF oscillator linewidth at 1 Hz, percentage of received power at 0.5 and RF signal input power at 20 dBm. Laser linewidth does not seem to be the dominant degradation factor as the change is very small i.e less than 0.01 dB. The laser linewidth has been chosen keeping in view the linewidth of commercially available Distributed feedback laser (DFB) and febryperot (FP) laser i.e. 10 MHz & 624 MHz. Transmission distance is selected in accordance with the range of radius of microcell which is 1 to 10 Km. An increment of 0.67 dB is observed in the peak value of SNR while adopting 120o Hybrid Coupler in comparison with 90o Hybrid Coupler. The effect RF oscillator is more prominent. This means that we can employ a cheap laser such as the FP laser in the RoF system in microcell without a severe SNR degradation. Fig. 1.6 shows the variation of SNR as a function of input RF signal power. The value for optical fiber length is kept at 8 km, RF oscillator linewidth at 1 Hz, percentage of received power at 0.5 and laser linewidth at 624 MHz. It can be seen that SNR increases as an increment in input RF signal power. SNR increases by almost 8.6 dB with increase in input RF signal power from 0 to 20 dB. Further, an increment of 0.5339 dB is observed in the peak
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Please cite this article as: P. Kumar, et al., Optics Communications (2015), http://dx.doi.org/10.1016/j.optcom.2015.09.081i