Experimental Demonstration of a Heterodyned Radio-over-fiber System using Unlocked Light Sources and RF Homodyning at the Receiver A.H.M. Razibul Islam1, Masuduzzaman Bakaul1, Ampalavanapillai Nirmalathas2, Lenin Mehedy1 and Graham Town3 1 NICTA Victoria Research Laboratory, Dept. of EEE, University of Melbourne, Australia Dept. of EEE, University of Melbourne, Australia, 3 Dept. of Physics and Engg., Macquarie University, Australia

DFB LD 1

-10 -20

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-30 -40 1548.5 1549.0 1549.5 1550.0 Wavelength (nm)

RF Power (dBm)

Optical Power (dBm)

Millimeter-wave (mm-wave) radio-over-fiber (RoF) systems have been considered as a promising solution in delivering broadband wireless services to customers [1]. Due to the nature of last mile access technology, costeffectiveness and simplicity of these systems are prerequisites for successful commercial deployment. Recent research on the generation and distribution of mm-wave RoF schemes can be classified into two groups. In the first group, an optical heterodyne technique is employed where frequency offset between two frequency/phase locked optical tones generates desired mm-wave carrier frequency for up-conversion of baseband signals. Coherence of the light sources determines the amount of phase noise in the generated mm-wave signals. Several techniques have been demonstrated to reduce phase noise effects which include optical phase locking or injection locking of two laser diodes [1-2], mode-locked lasers [2] and dual-mode lasers [3]. In these methods, complex locking arrangements and use of high frequency local oscillators (LO) at both central office (CO) and base

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II. THEORY OF OPERATION Fig. 1 presents our proposed RoF system configuration. Electric fields of two single mode lasers in our system can be represented as E1 exp j (2πυ1t + φ1 ) and E2 exp j (2πυ 2t + φ2 ) . E1 and E2 are the peak amplitudes of the electric fields and υ1 and υ 2 are optical frequencies of lasers. Assuming E1 = E2 = 1 , after double

(ii)

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10 20 30 Frequency (GHz)

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(iii)

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EA

PC

P=+15 dBm

LO OC

Mixer

MZM SMF EDFA

Optical Heterodyning

155 Mbps PRBS Data

RF Homodyning

(v)

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200 400 600 800 Frequency (MHz)

BERT LPF

50 GHz PD EA Phase Shifter

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Att

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I. INTRODUCTION

stations (BS) are essential to avoid phase noise effects. The second approach employs coherent spectral lines from a single light source to generate either optical single sideband with carrier (OSSB+C) or carrier-suppressed double sideband (CS-DSB) signals by using expensive and high-speed modulators, LOs and rigorous filtering arrangements [4]. Recently, we proposed a simplified architecture [5] and extended the concept in [6] where two off-the-shelf distributed feedback (DFB) light sources with different linewidths were optically heterodyned and later the photodetected mm-wave carrier was homodyned to effectively avoid the phase noise effects. In this paper, we validate the proposed concept experimentally. In this demonstration, a 155 Mbps pseudo-random-bit-sequence (PRBS) data was imposed on two unlocked light sources separated by 35.75 GHz, transported over 26.2 km of single mode fiber (SMF), detected and homodyned without the need for microwave synthesizers or LO’s and high-speed optical modulators.

RF Power (dBm)

Abstract—A simplified millimeter-wave radio-over-fiber system is demonstrated experimentally by heterodyning unlocked light sources and homodyning the RF signal at the receiver. The technique avoids phase/frequency locking, high-speed modulators and local oscillators at CO and BSs.

RF Power (dBm)

2

(iv)

-20 -40 -60 0

10 20 30 Frequency (GHz)

Fig. 1. Experimental setup for the proposed RoF system configuration (Inset (i) to (v) shows the relevant optical and RF spectra)

40

ip (t ) = ℜ ×





∑∑

n =−∞ p =−∞

j(

n− p)

J n ( m )J p ( m ) × exp j ( n 2 − p 2 )( 0.5β 2 zω 2 m )

{

× 2exp j ⎡⎣ 2π ( n − p ) f mt ⎤⎦ + exp j ⎡⎣ 2π {(υ1 − υ 2 ) + ( n − p ) f m } t + (φ1 − φ2 ) ⎤⎦

+ exp j ⎡⎣ 2π {(υ2 − υ1 ) + ( n − p ) f m } t + (φ2 − φ1 ) ⎤⎦

(1)

Here, ℜ is the responsivity of the photodetector (PD), J n ( x ) is the Bessel function of the first kind of order n and argument x . p is the conjugate terms for photodetection process. β 2 is the second-order fiber chromatic dispersion coefficient and z is the length of the fiber. m is the modulation index of the single-drive MZM, f m is the frequency of the modulated mm-wave signal. The second-to-the-last term in equation (1) clearly shows the desired mm-wave frequency at (υ1 − υ2 ) together with the harmonic components and

the phase noise (φ1 − φ2 ) as a result of beating of two lasers. Homodyning or self-mixing equation (1) also predicts higher order components. However, for digital signals these components have minimal impact in practice as the signal-to-noise-ratio would be higher enough to transmit higher quality data [7-8]. Furthermore, whilst frequency drift of the lasers shifts the RF carrier frequency, the drifts are cancelled at baseband by homodyning. III. EXPERIMENTAL SETUP AND RESULTS Fig. 1 shows the setup to demonstrate the proposed concept experimentally. Two unlocked DFB light-sources operating at 1549.214 and 1549.50 nm are combined with a 3 dB optical coupler (OC) that gives a channel separation of 35.75 GHz. Linewidths of the lasers were 5 MHz and 200 KHz at a relative intensity noise of -145 dBc/Hz. A binary phase-shift-keyed (BPSK) formatted 155 Mbps data with a PRBS pattern of 231 − 1 and amplitude of 2 V p − p is used to drive the MZM with a VΠ of 4.5 V. The DSB modulated optical output is then amplified by an erbium-doped-fiber-amplifier (EDFA), followed by an optical bandpass filter (OBPF) of 2 nm bandwidth, and transported to the BS with a 26.2 km of SMF. A high-speed 50 GHz PIN PD with responsivity of 0.65 A/W detects the desired mm-wave RF signal, together with a baseband replica. The signal is then electrically amplified in the range of 26.5 to 40 GHz, divided and homodyned using a Miteq mixer followed by a low pass filter (LPF) of 545 MHz bandwidth. An input power of +15 dBm is launched to the LO port of the mixer that turns the diodes on. A variable phase shifter is used before the RF port of the mixer to enable phasematched multiplication of the signals. The relevant optical and RF spectra are shown in the insets of Fig. 1. Insets (ii) and (iii) of Fig. 1 show the full band RF spectra after PD and after the amplifier while insets (iv) and (v) show full band RF spectra after LPF and recovered baseband data spectra after LPF respectively.

Log10(BER)

sideband modulated (DSB) by single drive MachZehnder modulator (MZM) and transmission through SMF, the photodetected output can be expressed as [6]

BTB After 26.2 km SMF

-4 -6 -8

i) BTB

-17.5

ii) After fiber

-17.0 -16.5 -16.0 -15.5 Received Optical Power (dBm)

-15.0

Fig. 2. BER curves for the proposed system

Fig. 2 shows the bit-error-rate (BER) curves and eye diagrams for both back-to-back (BTB) and after transmission over 26.2 km SMF conditions. The error free (at a BER of 10-9) recovery of data with a receiver sensitivity of -16.75 dBm confirms the functionality of the proposed concept experimentally. The incurred 0.2 dB power penalty is very negligible and can be attributed due to experimental errors. IV. CONCLUSIONS We have demonstrated that our proposed scheme using unlocked DFB lasers offers a simple and cost-effective realization of mm-wave RoF system and thus avoids phase/frequency locking, costly high-speed modulators and LOs/mixing circuitry in CO and BSs. This simple technique has the potential to be compatible with other access technologies such as passive optical networks (PON) and fiber-to-the-home (FTTH). REFERENCES. [1] A.J. Seeds, and K.J. Williams, “Microwave photonics”, J. of Light. Tech, USA, vol. 24, pp. 4628-4641, Dec. 2006. [2] J. Yao, “Microwave photonics,” J. of Light. Tech, USA, vol. 27, pp. 314-335, Feb. 2009. [3] D. Wake et al., “Optical generation of millimeter-wave signals for fiber-radio systems using a dual-mode DFB semiconductor laser,” IEEE Trans. on Micro. Theory and Tech., USA, vol 43, pp. 2270-2276, Sept. 1995. [4] M. Bakaul, A. Nirmalathas, C. Lim, D. Novak, and R. Waterhouse, “Spectrally efficient hybrid multiplexing and demultiplexing schemes toward the integration of microwave and millimeter wave radio-over-fiber systems in a WDM-PON infrastructure,,” J. of Optical Networking, USA, vol. 8, pp. 462-470, May 2009. [5] A.H.M.R. Islam, and G.E. Town, “A novel radio over fibre system using a dual-wavelength laser,” Photonics 2008, Delhi (India), Dec. 2008, pp. 1-4. [6] A.H.M.R. Islam, M. Bakaul, A. Nirmalathas, G.Town, “Simplified millimeter-wave radio-over-fiber system using optical heterodyning of low-cost independent light sources and RF homodyning at the receiver,” MWP 2009, Valencia (Spain), Oct. 2009, pp. 1-4. [7] T. Kuri, and K. Kitayama, “Optical heterodyne of millimeter-wave-band radio-on-fiber signals with a remote dual-mode local light source,” IEEE Trans. on Micro. Theory and Tech., USA, vol 49, pp. 2025-2029, Oct. 2001. [8] I. Garrett et al., “Impact of phase noise in weakly coherent systems: a new and accurate approach,” J. of Light. Tech., USA, vol. 8, pp.329-337, March 1990.

Experimental Demonstration of a Heterodyned Radio ...

155 Mbps PRBS Data .... (PRBS) data was imposed on two unlocked light sources separated by 35.75 GHz, ... fiber. m is the modulation index of the single-drive. MZM, m ... free (at a BER of 10-9) recovery of data with a receiver sensitivity of ...

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