A bidirectional 60 GHz RoF system based on FWM in a semiconductor optical amplifier

Optics Communications 283 (2010) 2238–2242

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A bidirectional 60 GHz RoF system based on FWM in a semiconductor optical amplifier Jing Li, Tigang Ning *, Li Pei, Chunhui Qi Institute of Lightwave Technology, Beijing Jiaotong University, Beijing 100044, China Key Lab of All Optical Network and Advanced Telecommunication Network of EMC, Beijing Jiaotong University, Beijing 100044, China

a r t i c l e

i n f o

Article history: Received 27 October 2009 Received in revised form 21 January 2010 Accepted 21 January 2010

Keywords: Optical communications Radio frequency photonics Four-wave mixing

a b s t r a c t In this study, a bidirectional 60 GHz RoF systems based on four-wave mixing (FWM) in semiconductor optical amplifier (SOA) is proposed. Two key techniques are included in such a scheme, namely, generation of 60 GHz modulating signals and distribution of 60 GHz local oscillator. The analytical model is theoretically analyzed and then confirmed by numerical simulations. Results of this study demonstrate that such a scheme can offer realistic solutions to support future mobile broad-band applications. Crown Copyright Ó 2010 Published by Elsevier B.V. All rights reserved.

1. Introduction Fiber-optical wireless RoF systems are currently being considered for the efficient delivery of multimedia services to wireless/ mobile users [1]. The license-free band, 60 GHz band, is seen as an ideal choice for mobile communication and many laboratory demonstrations and field trials have been performed on this band [2–4]. Recently, there are several reports on downlink mm-wave generation based on FWM in a semiconductor optical amplifier [5–8]. In order to realize full-duplex radio over fiber system, another technique, distribution of mm-wave local oscillator for uplink signals downconversion, is also important [9]. In this letter, we propose and analyze a SOA based bidirectional 60 GHz RoF system. Two key techniques, generation of 60 GHz modulating signals and distribution of 60 GHz local oscillator, are concerned. In the proposed system two copolarized pumps and a signal probe are applied to the SOA in center station (CS). When the input pump power is much greater than signal power, the variation in the saturated traveling-wave amplifier (TWA) gain for the pumps and input signal is slight and negligible. Thanks to the FWM in the SOA, new converted tones are generated at the output of the SOA. The unwanted FWM tones can be removed by Fiber Bragg Grating (FBG) and optical bandpass filter (OBPF), leaving one converted tone and original three tones located alternately. Then the output lightwaves are transmitted to base station (BS) through dispersive fiber, and a 25/50 GHz optical interleaver is used to separate four optical lightwaves into two parts, both of which are * Corresponding author. E-mail address: [email protected] (T. Ning).

with 60 GHz spacing. One part is with modulating data and the other is not, which means that generation and distribution of 60 GHz modulating signals and local oscillator are realized. 2. Analytical model and discussions The analytical model is presented in Fig. 1. The Mach–Zehnder intensity modulator (IM) is biased in its maximum transmission point, so there are no odd harmonics in the optical field Ein ( 1 X Ein ðtÞ ¼ E0 exp½j2pf0 t þ ju1 ðtÞ J 0 ðmÞ þ ð1Þn J 2n ðmÞ½expðj2p  2nftÞ

)

n¼1

þ expðj2p  2nftÞ ;

ð1Þ

where E0 denotes the amplitude of optical field, f0 represents the ejected center frequency of tunable laser TL1, u1(t) can be considered as the phase noise, Jn is the Bessel function of the first kind of order n, f is the frequency of electrical driving signal, and m = pVm/ Vp is the phase modulation index. We can adjust m to make sure that optical sidebands with an order higher than J2 can be neglected without significant error. Then we use a uniform FBG to remove optical carrier. The optical field can be further written as

Eout ðtÞ ¼ J2 ðmÞE0 fexp½j2pðf0 þ 2f Þt þ ju1 ðtÞ þ exp½j2pðf0  2f Þt þ ju1 ðtÞg;

ð2Þ

The two second-order sidebands can be used as the pump lightwaves. Then we use an Erbium-doped optical fiber amplifier (EDFA) (gain parameter r2) to amplify the pump lightwaves. The

0030-4018/$ - see front matter Crown Copyright Ó 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2010.01.041

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Fig. 1. Schematic setup of the FWM based bidirectional 60 GHz RoF system.

two pumps can be expressed as EA(t) = J2(m)rE0exp(jxAt + ju1) and EB(t) = J2(m)rE0exp(jxBt + ju1), respectively, where xA = 2p(f0  2f) and xB = 2p(f0 + 2f). The signal lightwave can be written as

E1 ðtÞ ¼ Es sin

pa  i

2

exp½j2pfs t þ ju2 ðtÞ;

ð3Þ

where Es are the amplitude of the optical field, ai denotes the PRBS data value (0 and 1 representing the ‘‘off” and ‘‘on” state), u2(t) can be considered as the phase noise of TL2, and fS is the ejected center frequency of tunable laser TL2 (fS  f0 = 50 GHz). A polarization control (PC) is used in the CS to make sure the pumps and signal have the parallel polarization direction. As the total input power is increased over some range, the amplifier becomes saturated, resulting in compression of the gain. In general, the degree of gain G (expressed in dB) compression is a transcendental function of the total input power Ptot (expressed in c dBm) as [10,11]: G ¼ aP tot , where a and c are experimentally determined constants. Indicated by the authors in Refs. [10,11], when the operation wavelength is with 1550 nm and bias current is with 180 mA, the constants parameters can be measured as a = 11 dB, and c = 0.8. In our scheme, the total input power Ptot is

Ptot ¼ 10log10 ðjEA ðtÞj2 þ jEB ðtÞj2 þ jE1 ðtÞj2 Þ h  i 2 pai ; ¼ 10log10 2J 22 ðmÞr2 E20 þ E2s sin 2

ð4Þ

As can be seen in Eq. (4), Ptot is affected by the modulating data

ai, which means that final SOA gain G is not constant. The situation can be changed if we use an EDFA to amplify the two pump lightwaves. When the power of pump lightwaves is at least 10 dB higher than that of signal lightwave, Eq. (4) can be further simplified as

Ptot  10log10 ½2J 22 ðmÞr2 E20 ;

ð5Þ

Therefore, Ptot can be assumed as constant and so do the SOA gain. The converted signal with frequency xA  xB + xS consists of two parts: one is beating frequency at xA  xB modulates the signal field xS and the other is beating xS  xB modulates the pump field at xA. The converted signal can be concluded as

EC ¼ EABS þ ESBA ¼ J 22 ðmÞE20 Es sin

pa  i

2

r2 d3 exp½jðxA  xB þ xS Þt

þ ju2 ðtÞ  ½rðxA  xB Þ þ rðxS  xB Þ;

ð6Þ

where r(xA  xB) and r(xS  xB) are the relative conversion efficiency coefficients, d denotes the gain of SOA (d2 = 10G/10). As the relative conversion efficiency coefficients are inversely proportional to the frequency spacing [12], r(xS  xB) >> r(xA  xB) is satisfied in our case. Therefore, the converted signal can be simplified as

EC ¼ J 22 ðmÞE20 Es sin

pa  i

2

r2 d3 rðxS  xB Þ exp½j2pðfs  4f Þt þ ju2 ðtÞ; ð7Þ

Our analysis is restricted to isotropic (bulk) materials without birefringence. Under these circumstances, the relative conversion efficiency coefficient r(xS  xB) can be expressed as |r(xS  xB)|2 = R(xS  xB), where R(xS  xB) denotes the relative conversion efficiency function [12]. When the frequency spacing Df = fS  fB = 20 GHz (corresponding Dk = 0.16 nm) is realized in our case. According to the experimental results of Ref. [12], R is approximately 40 dB. As can be seen in Fig. 1, the unwanted FWM tones can be removed by FBG and OBPF. The output optical field E3 is

pa  i E3 ðtÞ ¼ Es sin d exp½j2pfs t þ ju2 ðtÞ þ Es J 22 ðmÞE20 r2 d3 rðxS  xB Þ 2 pa  i exp½j2pðfs  4f Þt þ ju2 ðtÞ  J 2 ðmÞE0 rd  sin 2  fexp½j2pðf0 þ 2f Þt þ ju1 ðtÞ þ exp½j2pðf0  2f Þt þ ju1 ðtÞg; ð8Þ Then the optical field is launched into a signal-mode fiber (SMF), with the effect of chromatic dispersion, each optical frequency component travels through the fiber at a different velocity. As a result, the optical field becomes distorted and the optical pulse broadens [13]. The arriving time of corresponding frequency component of Eq. (8) can be expressed as s1 = zcD/f0  zcD(fs  f0)/f02, s2 = zcD/f0  zcD(fs  4f  f0)/f02, s3 = zcD/f0  2zcDf/f02, and s4 = zcD/f0 + 2zcDf/f02, where D is expressed in ps/nm km corresponding to frequency f0, and z in km. At the BS, we use a 25/50 GHz optical interleaver to separate the subcarriers with and without modulating data. When the fiber

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(a) 20

(b) 20 10

10

Pump

0 Signal 60GHz

-10

Power / dBm

Power / dBm

0

-20 -30 -40 -50

-10 -20 -30 -40 -50

-60

-60

-70

-70

-80 1549.2 1549.4 1549.6 1549.8

1550

-80 1549.2 1549.4 1549.6 1549.8

1550.2 1550.4

Wavelength / nm

1550

1550.2 1550.4

Wavelength / nm

Fig. 2. Spectrum of optical field: (a) E2(t) and (b) E3(t).

attenuation effect is not taking into considered, the output optical field E4(t) and E5(t) can be described as

pa  i d exp½j2pfs ðt  s1 Þ þ ju2 ðt  s1 Þ 2 pa  i exp½j2pðfs  4f Þ þ Es J 22 ðmÞE20 r2 d3 rðxS  xB Þ sin 2  ðt  s2 Þ þ ju2 ðt  s2 Þ;

E4 ðtÞ ¼ Es sin

ð9Þ



 exp½j2pðf0 þ 2f Þðt  s3 Þ þ ju1 ðt  s3 Þ E5 ðtÞ ¼ J 2 ðmÞE0 rd ; þ exp½j2pðf0  2f Þðt  s4 Þ þ ju1 ðt  s4 Þ ð10Þ As can be seen in Eqs. (9) and (10), nonstationary Gaussian stochastic process u(t) exist in optical fields E2(t) and E3(t). After photodetection, the photocurrents i1 and i2 are in direct proportion to the mean value of exp[ju1(t  s1)  ju1(t  s2)] and exp[ju2(t  s3)  ju2(t  s4)], respectively (responsivity of photodiode is 1A/W). Phase noise u(t) is nonstationary Gaussian stochastic process with mean value of 0 and variance value of 2pft, where f represents 3 dB linewidth of laser. Then u(t1)  u(t2) is stationary Gaussian stochastic process with mean value of 0 and variance value of 2pf(t1  t2). The mean value of exp[ju(t1)  ju(t2)] is

Efexp½juðt1 Þ  juðt2 Þg ¼

  1 x2 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi exp  4pfðt 1  t 2 Þ fðt 1  t2 Þ 1 2p   1  expðjxÞdx ¼ exp   2pf j t1  t2 j 2 Z

þ1

¼ expðpf j t 1  t 2 jÞ;

ð11Þ

The final photocurrents i1 and i2 can be expressed as 2

i1 ¼ E2S J 22 ðmÞE20 r2 d4 rðxS  xB Þ sin   4pf1 zcDf  exp  ; 2 f0

pa  i

2

cosð8pftÞ

  4pf2 zcDf i2 ¼ J 22 ðmÞE20 r2 d2 cos ð8pftÞ exp  f02

ð12Þ

ð13Þ

When electrical driving signal is with frequency of 15 GHz, the generated 60 GHz mm-wave signal and local oscillator are presented in Eqs. (12) and (13). Taking i1 for example, if f1 equals to 0, the power of 60 GHz signals reaches maximum value. However,

linewidth f1 cannot be limited to 0. The 60 GHz amplitude degradation due to phase noise is DA = exp(4pf1zcDf/f02). The performance of system is evaluated by bit rate error (BER) when baseband is modulated in the CS and demodulated in the BS. If the system noise is simplified to be additive white gaussian noise (AWGN), then SNR can be described by

1 Pe ¼ erfc 2

sffiffiffiffiffiffiffiffiffiffiffi! Pmm : 4n0 B

ð14Þ

where Pmm denotes the power of 60 GHz signal, n0 represents the statistic power spectral density of AWGN, and B is the electrical bandwidth of bandpass filter used in demodulation. 3. Simulation and results In order to verify the proposed scheme, simulations are implemented by a Matlab program. In the system, two tunable lasers TL1 and TL2 are operated at the wavelength of 1550 nm (frequency of f0 = 193.41 THz) and 1549.6 nm (frequency of fS = 193.46 THz), respectively. The power of tunable laser is E02 = 1 mw (0 dBm), E2S = 0.1 mw (10 dBm). The phase modulation index m is set as 2p/3. In our case, the SOA gain G can be figured out G  10.6 dB. The signal probe is the signal with intensity modulated 1 Gbit/s PRBS data. The RF electrical driving signal is with frequency of f = 15 GHz and the gain of EDFA is 10 dB. The dispersive fiber used in the system is with dispersion parameter D = 17 ps/nm km. The statistic power spectral density of additive white gaussian noise is assumed as n0 = 1e  19 W/Hz. The spectrum of E2(t) can be seen in Fig. 2a. The frequency spacing of the two pumps is 60 GHz. After the process of SOA, converted signal can be seen in Fig. 2b. At the BS, a 25/50 GHz optical interleaver is used to separate subcarriers into two parts, which are 60 GHz optical signals with and without modulated data. Increasing the linewidth of TL1 from 0 to 2 GHz, the relationship between signal power degradation and linewidth corresponding to different fiber length is shown in Fig. 3a. Then increasing the fiber length from 0 to 100 km, the relationship between signal power degradation and fiber length corresponding to different linewidth is shown in Fig. 3b. Note that with linewidth f increase, the mm-wave power decrease. When fiber length z equals to 50 km and f equals to 500 MHz, the power degradation is approximately 5.5 dB. When f is less than 100 MHz, the mm-wave power degradation is not serious. Therefore, the linewidth f should be kept small to maximum the power of mm-wave signal.

2241

(a)

0

Power Degradation / dB

J. Li et al. / Optics Communications 283 (2010) 2238–2242

10

(b) 0 Power Degradation / dB

5

15 20

5

10

25 30

15

35

z=25km z=50km z=75km z=100km

40 45 50 0

=100MHz =250MHz =500MHz =1GHz

20

0.2 0.4 0.6 0.8

1

1.2 1.4 1.6 1.8

ζ / GHz

25 0

2

10

20

30

40

50

60

70

80

90 100

z / km

Fig. 3. Simulation results: (a) power degradation verse f and (b) power degradation verse z.

(a) 100

Bit Error Rate

10 10 10 10 10 10 10

-10

10

-20

10

Bit Error Rate

10

(b)100

-30

-40 -50

z=25km z=50km z=75km z=100km

-60 -70

-80

0

0.2 0.4 0.6 0.8

1

1.2 1.4 1.6 1.8

ζ 1 / GHz

10 10 10 10 10 10

2

-10

-20

-30

-40 -50

1=100MHz 1=250MHz 1=500MHz 1=1GHz

-60 -70

-80

0

10

20

30

40

50

60

70

80

90

100

z / km

(c)100 -50

Bit Error Rate

10

-100

10

-150

10

-200

10

250Mbit/s, 25km 500Mbit/s, 25km 750Mbit/s, 25km 1Gbit/s, 25km

-250

10

-300

10

0

0.2 0.4 0.6 0.8

1

1.2 1.4 1.6 1.8

ζ1 / GHz

2

Fig. 4. BER performance: (a) BER verse f1 corresponding to different z; (b) BER verse z corresponding to different f1 and (c) BER verse f1 corresponding to different bit rate.

The system performance is also related to linewidth f1. We can also simulate the process of PRBS data transmission. Ignoring nonlinear effect and attenuation effect of SMF, we only consider the performance degradation induced by linewidth. We simulate the BER performance due to laser linewidth in different system parameter, as can be seen in Fig. 4a–c. The simulation results show that with laser’s linewidth increase, the BER performance degraded a lot. However, when linewidth is adjusted less than 100 MHz, the BER performance degradation is not serious. Therefore, using narrow-linewidth lasers (f1 less than 100 MHz) as sources can be considered to be the best way to improve the BER performance of 60 GHz bidirectional RoF system.

Acknowledgements This work is jointly supported by the National Natural Science Foundation of China(60771008, 60837002), Beijing Natural Science Foundation (4082024), the Ph.D. Programs Foundation of Ministry of Education of China (20090009110003), and the Foundation for the Returning Scholars (2008890). References [1] M. Sauer, A. Kobyakov, J. George, J. Lightwave Technol. 25 (2007) 3301. [2] R.R.C. Office, Detailed Spectrum Investigation-First Phase: 3400 MHz to 105 GHz, 1993, p. 27.

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