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Optical Single Sideband Modulation of 9-GHz

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918 PIERS Proceedings Suzhou China September 1216 2011 Optical Single Sideband Modulation of 9GHz RoF System Based on FWM Eects of SOA P H Hsie1 W S Tsai1 C C Weng1 and H H Lu2 1Department of Electrical Engineering Ming Chi University of Technology 84 Gungjuan Rd Taishan Taipei 24301 Taiwan 2Department of ElectroOptical Engineering National Taipei University of Technology 1 Sec3 ChungHsiao E Rd Taipei 10608 Taiwan ROC Abstract We propose an optical single sideband OSSB modulation scheme using se......

918

PIERS Proceedings, Suzhou, China, September 12–16, 2011

Optical Single Sideband Modulation of 9-GHz RoF System Based

on FWM Eﬀects of SOA

P. H. Hsie

W. S. Tsai

C. C Weng

and H. H. Lu

Department of Electrical Engineering, Ming Chi University of Technology

84 Gungjuan Rd., Taishan, Taipei 24301, Taiwan

Department of Electro-Optical Engineering, National Taipei University of Technology

1, Sec.3, Chung-Hsiao E. Rd., Taipei 10608, Taiwan, R.O.C.

Abstract—

We propose an optical single sideband (OSSB) modulation scheme using self-phase

modulation (SPM), cross-phase modulation (XPM) and four-wave mixing (FWM) eﬀects of SOA

to achieve wavelength conversion. Drive with the 9 GHz RF signal into electro-absorption mod-

ulator laser (EML) and IM modulator. By properly adjust the phase shifter for phase diﬀerence

between the two paths of electrical signal. FWM signal is OSSB format for a 25 km SMF trans-

mission. Finally, we use an optical spectrum analyzer (OSA) to observe optical spectrum.

1. INTRODUCTION

In traditional intensity modulation, the optical carrier is modulated to generate an optical signal

with double sideband (DSB) format. DSB modulation produces two sidebands on both sides of the

carrier, and the beating components between the carrier and two sidebands induce interference at

the receiver. Over a long haul ﬁber transmission, RF signal will cause severe power degradation

due to chromatic dispersion [1]. This phenomenon will degrade systems’ performance. In order to

overcome RF power degradation due to ﬁber dispersion, optical single sideband (OSSB) modulation

technique must be implemented [2]. The fact that OSSB modulation can remove a half of the

optical spectrum is expected to attain a dispersion beneﬁt because the optical spectrum has been

reduced by a factor of two. Several OSSB generation methods [3–6] have been proposed. By

eliminating one of the sidebands, OSSB modulation not only immunizes to ﬁber dispersion, but it

also increases the spectral eﬃciency twice. One way for generating OSSB signals is to exploit the

Hilbert transform [7]. Using a dual electrode Mach-Zehnder modulator can be reality to generate

OSSB signal [8]. Another way to generate OSSB signals is to utilize narrow optical ﬁlter. The

narrow optical ﬁlter at the end of the ODSB transmitter output can eliminate one of the sidebands

to achieve OSSB modulation [9]. Optical ampliﬁer plays an important role in a long haul ﬁber

transmission system, the use of semiconductor optical ampliﬁer (SOA) as an optical ampliﬁer is very

attractive since it can potentially be used to help upgrade ﬁber penetration, easily integrated and

small compact. However, SOA also has several nonlinearity eﬀects such as self-phase modulation

(SPM), cross-phase modulation (XPM), self-gain modulation (SGM), cross-gain modulation (XGM)

and four-wave mixing (FWM) that will degrade systems’ performances. In this letter, we propose

an OSSB modulation scheme using SPM, XPM and FWM eﬀects of SOA to achieve wavelength

conversion. The translation wavelength possessing OSSB modulation format can prevent ﬁber

dispersion induced RF signal fading. Using the phase modulation eﬀects in SOA to generate

OSSB format is relatively simple to implement. It only needs SOA and electrical phase shifter to

construct the OSSB system instead of a complex circuit or device to exploit the Hilbert transform

of the conventional OSSB systems.

2. EXPERIMENTAL SETUP

The experimental system conﬁguration of our proposed OSSB system to operate wavelength conver-

sion based on SPM, XPM and FWM eﬀects of SOA associated with electrical phase shifter is shown

in Fig. 1. The main parts of transmitter consists of a tunable laser source as local oscillation light,

a polarization controller (PC), a Mach-Zehnder modulator (MZM), a microwave signal generator

with 9 GHz, a RF power splitter, a phase shifter, a RF power attenuator and an electro-absorption

modulator laser (EML).

The MZM input light source is 1549.6 nm provided by a tunable laser source, because the optical

modulator has the sensitive characteristic regarding the input photo source’s polarization state. A

PC needs to be added on in front of the MZM input section. This function is to maintain the

input light at the speciﬁc polarization state when the light launches into the optical modulator.

Progress In Electromagnetics Research Symposium Proceedings, Suzhou, China, Sept. 12–16, 2011

919

9 GHz RF signal is generated by a microwave signal generator and fed into a RF power splitter to

split two copies. One copy of RF signal passes through a phase shifter and adjusts optimal power

with tunable RF power attenuator, then feeds into optical modulator. Another one copy of the RF

source supplies to EML as modulating signal. Two paths of optical light are combined by an optical

coupler. Passing through an optical power attenuator to adjust the suitable input power avoids the

SOA output power saturation. Output signal launches into the band-pass ﬁlter to ﬁlter out the

noise which provided by SOA. For a 25 km SMF transmission, we use an optical spectrum analyzer

(OSA) to measure optical spectrum. On the other hand, PD transforms optical signal to electrical

one, and measures the output frequency spectrum by using an electrical spectrum analyzer (ESA).

Figure 1: Experimental setup of OSSB modulation based on SPM, XPM and FWM eﬀects of SOA.

(a)

(b)

(c)

(d)

Figure 2: Conceptual diagram of (a) OSSB modulation system for wavelength conversion technique, (b) op-

tical spectrum before passing through SOA, (c) optical spectrum after passing through SOA, (d) ﬁltering

out

FWM

and adjusting parameters of system achieved OSSB signal.

920

PIERS Proceedings, Suzhou, China, September 12–16, 2011

3. RESULTS AND DISCUSSION

Figure 2(a) shows a conceptual diagram of OSSB modulation system for wavelength conversion

technique using SOA and electrical phase shifter. Two optical signals,

EML

and

are launched

into SOA as shown in Fig. 2(b).

After passing through SOA, the FWM eﬀect causes new optical signal (λ

FWM

) to appear shown

in Fig. 2(c). In general situation, FWM is a bad noise disturbance item. However, this kind of

non-linear eﬀect can easily perform wavelength conversion. Filter out FWM and adjust parameters

of system can realize OSSB system shown in Fig. 2(d).

In the case of a single RF tone modulation, the OSSB optical ﬁeld can be written as [10]

E(t)

sin(ω

exp

i ω

sin(ω

)

sin(ω

) +

sin(ω

γ)

(1)

where

denotes the optical carrier frequency,

is the subcarrier frequency, and

is modulation

index.

k, α

, and

denote modulation decrease factor, prechirp parameter, SPM chirp

parameter, phase diﬀerence of prechirp from AM modulating term and phase diﬀerence of SPM of

SOA from AM modulating term, respectively.

is a chirp parameter which is relevant to XPM,

can be any value between 0 and 2π.

In our experiment, we launched electrical source at 9 GHz to EML and IM modulator. The

wavelength of optical wave from EML was 1550.8 nm. EML was biased to

−0.1

V, and the operation

current was 75 mA. SOA operation current was 320 mA. Fig. 3 shows the optical spectrum in back

of the optical coupler. Because 9 GHz RF signal is fed into EML and IM modulator, we can observe

two optical carriers with DSB modulation format.

By the nonlinear eﬀects of SOA, the optical spectrum can be observed in Fig. 4. In the Fig. 4,

FWM

appears in the right side and it also has DSB modulation format. The FWM eﬃciency is

deﬁned as the FWM signal power at the SOA output divided by signal power at EML. When the

input signal power is too much, the converted signal power decreased, which is due to the gain

saturation of SOA. The conversion eﬃciency in our experiment is about

−20

dB.

Figures 5 and 6 show the

FWM

from DSB format transformed to SSB format, when we tune

the phase shifter and the operation current of SOA. We tune the phase shifter to adjust the phase

diﬀerence between the two ways of electrical signal. When two orthogonal waves are modulated

at the same single tone RF carrier, propagate in SOA. Due to SPM and XPM eﬀects of SOA, the

FWM

can reveal SSB modulation. In this way, we can observe the transformation of modulation

format by tuning phase shifter.

Figure 3: Measured optical spectrum in back of the

optical coupler.

Figure 4: Measured optical spectra based on nonlin-

ear eﬀects of SOA.

Progress In Electromagnetics Research Symposium Proceedings, Suzhou, China, Sept. 12–16, 2011

921

Figure 5:

FWM

from DSB format transformed to

SSB one at the upper sideband.

4. CONCLUSION

Figure 6:

FWM

from DSB format transformed to

SSB one at the lower sideband.

We propose an OSSB modulation scheme using SPM, XPM and FWM eﬀects of SOA to achieve

wavelength conversion. Using this method to construct the OSSB system can substitute a com-

plex circuit or device to exploit the Hilbert transform of the conventional OSSB system. In the

future work, we can enhance the wavelength conversion eﬃciency and transmit digital signal in our

proposed OSSB system.

REFERENCES

1. Chi, H. and J. P. Yao, “Frequency quadrupling and upconversion in a radio over ﬁber link,”

J. Lightwave Technol.,

Vol. 26, 2706–2711, 2008.

2. Hong, C., C. Zhang, M. J. Li, L. X. Zhu, L. Li, W. W. Hu, A. S. Xu, and Z. Y. Chen, “Single-

sideband modulation based on an injection-locked dfb laser in radio-over-ﬁber systems,”

IEEE

Photon. Technol. Lett.,

Vol. 22, 462–464, 2010.

3. Blais, S. R. and J. P. Yao, “Optical single sideband modulation using an ultranarrow dual-

transmission-band ﬁber bragg grating,”

IEEE Photon. Technol. Lett.,

Vol. 18, 2230–2232, 2006.

4. Fonseca, D., A. V. T. Cartaxo, and P. Monteiro, “Adaptive optoelectronic ﬁlter for improved

optical single sideband generation,”

IEEE Photon. Technol. Lett.,

Vol. 18, 415–417, 2006.

5. Sung, H. K., E. K. Lau, and M. C. Wu, “Optical single sideband modulation using strong

optical injection-locked semiconductor lasers,”

IEEE Photon. Technol. Lett.,

Vol. 19, 1005–

1007, 2007.

6. Sun, X. Q., K. Xu, S. N. Fu, J. Q. Li, X. B. Hong, J. Wu, J. T. Lin, and P. Shum, “All-optical

WDM subcarrier modulator for binary phaseshift keying (BPSK) with optical ssb format using

aphase modulator loop mirror ﬁlter,”

Conference on Opt. Fiber Communication,

2009.

7. Takano, K., N. Hanzawa, S. Tanji, and K. Nakagawa, “Experimental demonstration of optically

phase-shifted SSB modulation with ﬁber-based optical hilbert transformers,”

Conference on

Opt. Fiber Communication,

1–3, 2007.

8. Hou, C., Y. Shao, X. Liu, X. Zheng, X. Li, S. Zou, and N. Chi, “Dual-level optical single

side band modulation scheme for 0.1 tera Hz radio-over-ﬁber systems,”

Communications and

Photonics Conference,

1–6, 2009.

9. Fonseca, D. D., A. V. T., Cartaxo, and P. P. Monteiro, “Opto-electrical ﬁlter for 40 Gb/s

optical single sideband signal generation,”

Conference on Opt. Fiber Communication,

2006.

10. Lee, U.-S., H.-D. Jung, and S.-Kook, “Optical single sideband signal generation using phase

modulation of semiconductor optical ampliﬁer,”

IEEE Photon. Technol. Lett.,

Vol. 16, 1373–

1375, 2004.

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