Chromatic Dispersion Induced Optical Phase

1

Chromatic Dispersion Induced Optical Phase Decorrelation in a 60 GHz OFDM-RoF System Tong Shao, Eamonn Martin, Prince M. Anandarajah, Senior Member, IEEE, Colm Browning, Vidak Vujicic, Roberto Llorente, Member, IEEE, and Liam P. Barry, Senior Member, IEEE  Abstract—The authors propose and demonstrate a 25 Gb/s 16 quadrature amplitude modulation (16QAM) orthogonal frequency division multiplexing (OFDM) 60 GHz radio over fiber (RoF) transmission system based on a gain-switched optical comb source. The impact of the phase noise caused by the optical phase decorrelation due to the chromatic dispersion, is theoretically studied and experimentally investigated by employing a high linewidth comb source and transmitting over a 50 km standard single-mode fiber (SSMF) reel. A time delay pre-compensation is deployed based on the study of phase noise in order to reduce the phase noise impact, and thereby improve system performance. Index Terms—radio over fiber, gain-switching, phase noise, chromatic dispersion.

I. INTRODUCTION he demand for high data rate wireless connectivity is rapidly growing. 60 GHz technology has attracted much attention in the applications of high speed wireless communications because of the unlicensed spectral band of 7 GHz around 60 GHz that is available in most countries [1]. Due to the enormous propagation loss at 60 GHz, the typical transmission distance for these signals ranges from a few meters to tens of meters. Radio over Fiber (RoF) technology has raised great interest in the last decade due to its capability of providing optical transmission of radio signals to numerous simplified base stations (BSs) [2]. Another advantage associated with RoF technology is the fact that broadband optical signal processing functions can be centralized, therefore limiting the use of expensive broadband electronics in the BSs. Coherent heterodyning is a common photonic technique used to generate low phase noise millimetre wave (mmW) signals by beating two coherent optical tones. One of the conventional approaches for coherent optical tone (comb) generation is based on mode-locked lasers (MLLs) [3]. However, it does not offer free spectral range (FSR) tunability as the comb line spacing is fixed by the cavity length of the laser. Optical two-tone or multi-tone generation using external optical modulators is another approach that has been reported, where the FSR and the central wavelength of the comb can both be varied [4]. Different solutions have been proposed based on the

T

Manuscript received May 20, 2014. This work was supported in part by the SFI PI grant 09/IN.1/I2653 and 10/CE/I1853, the HEA PRTLI 4 INSPIRE Programs. Tong Shao, Eamonn Martin, Colm Browning, Vidak Vujicic, Prince M. Anandarajah, and Liam P. Barry are with the Radio and Optical Communication Lab, Rince Institute, Dublin City University, Ireland (Email: [email protected]). Roberto Llorente is with the Valencia Nanophotonics Technology Center, Universitat Politècnica de València, 46022 Valencia, Spain.

external modulation technique [5, 6]. However, the large insertion loss of the modulator (especially when cascaded), coupled with the modulation efficiency and the instability induced by bias drift, can prove prohibitive. Previously, we reported on the use of gain-switching to generate an optical comb [7]. Such a comb source enables simple and cost efficient generation of lightwaves with precisely controlled channel spacing, and offers high phase coherence between the optical tones. Fiber chromatic dispersion can partially destroy the phase correlation between two optical tones generated either by a comb source or by an optical two-tone generator based on external modulation, since it introduces a time delay between different optical tones [8]. In [8], a phase noise suppression (PNS) algorithm [9] is applied to reduce the phase noise. However, this will increase the complexity of the digital signal processing (DSP). Additionally, even when the PNS algorithm is applied, a residual 2 dB power penalty is still incurred due to the chromatic dispersion induced phase de-correlation [9]. Previously, we reported on a 60 GHz RoF system using a gain-switched laser [10]. The two optical tones, with 60 GHz frequency separation, were split into two optical paths using an arrayed waveguide grating. The phase noise induced by the different optical paths was theoretically analyzed and experimentally investigated. In this paper, we propose a 60 GHz orthogonal frequency division multiplexing (OFDM) RoF system. The phase noise of the 60 GHz signal due to the chromatic dispersion caused optical phase decorrelation is theoretically studied and experimentally investigated by employing a gain-switched laser with a linewidth of 60 MHz, and by transmitting over 50 km of standard single-mode fiber (SSMF). A time delay pre-compensation is deployed in the proposed system to mitigate the chromatic dispersion induced phase decorrelation. The paper is organized as follows. In section II the phase noise induced by the optical phase decorrelation due to the chromatic dispersion is presented and theoretically analyzed. Section III presents the experimental investigation. A 60 GHz OFDM RoF system is proposed which supports time delay pre-compensation. 25 Gbps 16QAM OFDM 60 GHz signal generation and transmission over 50 km SSMF is experimentally investigated. Finally our conclusions are presented in Section IV. II. THEORETICAL ANALYSIS OF CHROMATIC DISPERSION INDUCED PHASE NOISE IMPACT ON A 60 GHZ OFDM SIGNAL 

50km SSMF

Two-tone generator Fig. 1. Principle of photonic generation of a 60 GHz signal.

PD

2 point n as parameter B. As discussed in [10], B is only dependent on the time delay τ0. Thus it is possible to identify the time delay τ0 between the two channels by parameter B.

(2)

Fig. 3 shows the simulated constellations of a 16 quadrature amplitude modulation (16QAM) OFDM signal distorted by the phase noise with different lengths of SSMF using equations (3) and (6). The linewidth of the laser is set to 60 MHz and the signal-to-noise ratio (SNR) of the electrical OFDM signal is 15 dB. It shows the phase noise impact becomes significant as the length of the SSMF increases. In practice, gain-switched lasers which are used to generate optical frequency combs with higher FSRs require a short cavity length resulting in higher modulation bandwidth, which is not compatible with low linewidth. Thus, the phase noise induced by the optical phase decorrelation due to chromatic dispersion may substantially degrade the 60 GHz RoF system.

ET  t   E1  t   E2 t   0 

The delay between two optical tones induced by the chromatic dispersion is (3)  0  DL where D is the dispersion parameter which is 17 ps/km·nm (SSMF at 1550 nm), L is the length of the fiber and ∆λ is the wavelength offset of the two optical tones which is 0.48 nm, corresponding to 60 GHz (at 1550 nm). The photocurrent at the PD output can be calculated as I  t   ET  t  ET*  t  (4)  E12  E22  2 E1 E2 cos  2 f RF t  OP  t , 0   with

Relative to carrier power [dBc]

Fig. 1 shows the schematic associated with the generation of a 60 GHz signal and transmission over fiber. The optical tones are generated by an optical two-tone generator, which can be an optical comb source based on a gain-switched DFB laser or external modulators. The two optical tones are phase correlated and the optical fields can be expressed as E1  t   E1 exp  j  2 f1t  OP  t   (1) E2  t   E2 exp  j  2 f 2t  OP  t    The two optical tones are sent to the photodetector (PD) over a span of SSMF. Due to chromatic dispersion, the fiber transmission induces a time delay (τ0) between the two optical tones. Thus the optical signal ET(t) at the PD can be expressed as

10

10km SSMF 20km SSMF 40km SSMF 50km SSMF

0 -10 -20 -30 -40

B=5GHz m

-50 -60

56

n

58 60 62 Frequency [GHz]

64

Fig. 2. Simulated spectra with different lengths of SSMF.

OP  t , 0   OP  t   OP  t   0 

f RF  f1  f 2 where ∆OP(t) is the total phase jitter, or the resultant phase noise of the mmW signal, which is induced by the random optical phase change between t and t+τ0. As discussed in [11], the power spectral density (PSD) of the photocurrent (SI(f)) is calculated as  2  f  f RF   S I  f   E12 E22 4  2 f   exp  2 OP  0     2 OP  

(a) BTB (EVM=10.8%)

(b) 10km SSMF (EVM=15.4%)

   OP sin  2  f  f RF   0   cos  2  f  f RF   0   exp  2 OP  0     f  f RF    2  OP exp  2 OP  0    2  2   OP   2  f  f RF     

(5) where γOP is the full linewidth of the light source. Moreover, the variance of the resultant phase noise of the mmW signal can be expressed as [11]:  2OP  t , 0   2 OP  0 (6) Assuming one of the optical tones carries the OFDM signal, an OFDM mmW signal can be generated by the beating of the two optical tones. A simulation of the spectrum of the mmW signal taking into account the resultant phase noise impact can be undertaken using equation (3) and (5). Fig. 2 shows a simulated RF spectrum of the beat signal at 60 GHz with different lengths of SSMF based on equations (3) and (6). The length of the SSMF is set to 10, 25, 40 and 50 km respectively, and the linewidth of the optical tones is set to 60 MHz (corresponding to the linewidth of the optical comb used in the experiment). In Fig. 2, we define the frequency spacing between point m and

(c) 25km SSMF (EVM=20.4%) (d) 50km SSMF (EVM=25.6%) Fig. 3. Simulated constellations with different lengths of SSMF.

III. EXPERIMENTAL INVESTIGATION OF THE 60 GHZ OFDM ROF SYSTEM BASED ON A GAIN-SWITCHED LASER In this section, to validate the numerical model and results presented, we experimentally demonstrate an OFDM RoF system using a high linewidth comb source. The impact of the phase noise due to the fiber dispersion induced optical phase decorrelation is verified. Pre-compensation for the time delay induced by chromatic dispersion is implemented in the system in order to reduce the impact of phase noise. A. Experimental Setup Fig. 4 shows the experimental setup. In the central station (CS), the distributed feedback (DFB) laser is gain-switched with the aid of a 24 dBm, 18.1 GHz, RF signal (LO1,

3 AWG

CS

RF AMP1

Output

RF AMP2 Output

DEMUX

PC

Ch1

IB

MZM

EDFA 1

EDFA 2 OBPF

WSS

Tunable delay line PC Ch2

A

LO1, fLO1=18.1 GHz

VOA

BTB

LO2, fLO2=54.3 GHz

Off-line DSP

RTO ESA

BPF

LO IF

RF AMP3

RF

OBPF

PD VOA

50km SSMF

BS

EDFA 3 VOA

Fig. 4. Experimental setup.

fLO1=18.1 GHz) [7]. Therefore, a comb source with a FSR of 18.1 GHz is generated. The optical linewidth of each individual comb line is measured (using the standard delayed self-heterodyne technique) to be 60 MHz. A wavelength selective switch (WSS) is used to select two comb lines with a spectral spacing of 54.3 GHz at the transmitter. The two optical comb lines are amplified by an Erbium doped fiber amplifier (EDFA) with a gain of 27 dB, and are then separated into two individual paths by passing them through a 100-GHz DEMUX. Since the bandwidth of the DEMUX employed in the experimental setup is too large (over 80 GHz), two optical bandpass filters (OBPFs) are employed in order to reject the undesired comb lines in the two optical channels respectively. It is anticipated that the two optical filters can be removed if a 50 GHz WDM DEMUX is employed in the system. The optical comb line selected by channel 1 (Ch1) is modulated with an intermediate frequency (IF) OFDM signal by a dual-drive Mach-Zehnder modulator (DD-MZM), which is biased at the null point, resulting in double sideband carrier suppressed (DSB-SC) modulation. The 25 Gb/s IF 16QAM OFDM signals at 4.8 GHz (fIF=4.8 GHz) and its inverted copy are generated by an arbitrary waveform generator (AWG) with a sampling rate of 25 GSa/s, amplified by two RF amplifiers (AMP1 and AMP2) and applied to the two arms of the DD-MZM. In channel 2 (Ch2) of the DEMUX, a tunable delay line is employed to compensate for the time delay induced by the two optical channels of the DEMUX, as well as the time delay induced by the SSMF. A variable optical attenuator (VOA) is used to equalize the optical power in the two channels in order to enhance the optical beating efficiency. A polarization controller (PC) is employed to match the polarizations of the optical signals in the two channels. The two optical signals are recombined and amplified by another EDFA (EDFA2) with a gain of 21.5 dB, and an optical band pass filter (OBPF) is subsequently used to reject amplified spontaneous emission (ASE). The optical signal is then sent to the BS via 50 km of SSMF. The backto-back (BTB) case was also examined. The optical power launched into the 50 km of SSMF is maintained at 3 dBm in order to minimize the fiber non-linearity impact. In the BS, the optical signal is sent to EDFA3 via a VOA which is employed to vary the input optical power to the EDFA3. The output power of EDFA3 is maintained at 13 dBm. An OBPF is again used to reject ASE. The amplified optical signal is then sent to the PD via a VOA, ensuring that the optical power falling on the PD is maintained at -2 dBm. As a result of the photodection in the BS, the output of the PD contains the desired OFDM signal at 3fLO1 + fIF = 59.1 GHz and an undesired component at 3fLO1 - fIF =

49.5 GHz. A 56.2 GHz to 62 GHz RF band pass filter (BPF) is employed to suppress the undesired component. The 59.1 GHz OFDM signal is then amplified by an electrical amplifier (AMP3) with a gain of 28.7 dB. In order to demodulate the OFDM mmW signal, it is initially downconverted to an IF using an external mixer which is driven by a LO signal (LO2) at 54.3 GHz. Thus, the 59.1 GHz OFDM signal is down-converted to 4.8 GHz. The LO1 and LO2 are phase locked with each other in order to synchronize the carrier frequency between LO2 and the resultant 60 GHz mmW in the RoF system. In practice, the phase locking could be avoided by implementing carrier frequency estimation in the DSP. In this experiment, we phase locked the two LOs which alleviated the need for carrier frequency estimation in our off-line DSP code. The down-converted signal is captured by the real time oscilloscope (RTO) with a sampling rate of 50 GS/s. Offline DSP with Matlab including down-conversion, time synchronization, channel estimation and initial phase correction, is applied to demodulate the IF OFDM signal. The error vector magnitude (EVM) and bit error rate (BER) are also calculated in the DSP process. B. Experimental results The spectra of the unmodulated 59.1 GHz signal which is down-converted to 4.8 GHz, in the BTB case, and also with and without the optical length compensation over 50 km fiber transmission are shown in Fig. 5. The 54.3 GHz signal, which is generated by the beating of the two optical tones before splitting (point A in Fig. 4) and down-converted to 4.8 GHz, is shown in Fig. 5 as well. The blue line in Fig. 5 (a) and (b) show the spectra of the 59.1 GHz signal in the BTB case. It can be seen that the phase noise of the beat tone is low and similar to that for the beat tone at 54.3 GHz before the split (point A in Fig. 4). The electrical spectrum of the 59.1 GHz beat tone with the 50 km SSMF transmission is shown as the black line in Fig. 5 (a) and (b). Comparing the black and blue lines, it can be seen that the optical phase decorrelation due to the chromatic dispersion induces significant phase noise on the 59.1 GHz beat tone. The parameter B is 5.05 GHz, which agrees with the simulation result shown in Fig. 2 and corresponds to 400 ps time delay. The time delay induced by chromatic dispersion over 50 km of SSMF can be estimated as 408 ps using equation (3), which corresponds with the measured value. The spectra of the 59.1 GHz signal with precompensation using the optical tuneable delay line are shown as green curves in Fig. 5 (a) and (b). The phase noise of the 60 GHz signal can be effectively reduced by precompensating the time delay between the two optical tones.

-10 -20 -30

-20 -40

-40 B=5.05GHz

-50 -60 -70

BTB with compensation 50km SSMF without compensation 50km SSMF with compensation beat tone before split

0

2

4 6 Frequency [GHz]

8

10

24

10

BTB 50km SSMF

BTB 50km SSMF -2

22

10

20 18

FEC Limit

-3

10

-60

16

-80

0

-1

26

BER

BTB with compensation 50km SSMF without compensation 50km SSMF with compensation beat tone before split

EVM [%]

10 0



4

0

0.2 0.4 0.6 0.8 1 Frequency offset to carrier [MHz]

(a) (b) Fig. 5. Spectra of the down-converted unmodulated mmW signal.

Fig. 6 shows the constellations of the 16QAM-OFDM signals in the BTB case, and then over 50 km SSMF with and without optical length compensation. The corresponding EVM and BER are listed in Table I. Observing Fig. 6 (b), it can be seen that the phase noise caused by the optical phase decorrelation due to the chromatic dispersion can seriously distort the OFDM signal if a high-linewidth comb source is employed. Fig. 6 (c) shows the constellation with precompensation for the chromatic dispersion induced time delay. It is evident that this pre-compensation can mitigate the chromatic dispersion induced phase noise impact.

14 -30 -28 -26 -24 -22 -20 -18 -16 -14 Received optical power [dBm]

-4

10 -30 -28 -26 -24 -22 -20 -18 -16 -14 Received optical power [dBm]

(a) (b) Fig. 7. EVM and BER of the 25 Gb/s 16QAM-OFDM signal as a function of received optical power.

IV. CONCLUSION In this paper, a 60 GHz OFDM RoF system based on a gain-switched laser comb source is experimentally investigated. Phase noise induced by the optical phase decorrelation due to the chromatic dispersion is examined. Since the two optical comb lines are split into different optical channels in the CS, the fiber dispersion induced optical phase decorrelation can be fully cancelled by employing time delay pre-compensation. Transmission of a 25 Gb/s 16QAM OFDM 60 GHz signal over 50 km SSMF using a 60 MHz linewidth comb source is experimentally demonstrated. BER measurements as low as 8.3×10-4 are achieved. No power penalty is induced by phase noise if the time delay pre-compensation is deployed in the system. REFERENCES S.K. Yong, P. Xia, and A. Valdes-Garcia, “60 GHz Technology for Gb/s WLAN and WPAN: From Theory to Practice,” John Wiley & Sons, Ltd, 2011. [2] Z. Jia, J. Yu, G. Ellinas, and G.K. Chang, “Key enabling technologies for optical-wireless networks: Optical millimeter-wave generation, wavelength reuse, and architecture,” J. Lightw. Technol., vol. 25, no. 11, pp. 3452-3471, Jun. 2007. [3] F. Brendel, J. Poette, B. Cabon, T. Zwick, F. van Dijk, F. Lelarge, and A. Accard, "Chromatic Dispersion in 60 GHz Radio-over-Fiber Networks based on Mode-Locked Lasers," J. Lightw. Technol, vol. 29, no. 24, pp. 3816-3816, Oct. 2011. [4] T. Sakamoto, T. Kawanishi and M. Izutsu, “19x10-GHz Electro-Optic Ultra-Flat Frequency Comb Generation Only Using Single Conventional Mach-Zehnder Modulator”, CLEO, CMAA5, (2006). [5] L. Zhang, B. Liu, X. Xin, and Y. Wang, "MAMSK-OFDM Signal for RoF Access With Increased Tolerance Toward Frequency Offset," IEEE Photon. Technol. Lett., vol.25, no.4, pp.397-400, Feb. 2013. [6] L. Tao, J. Yu, Y. Wang, J. Zhang, X. Li, Y. Shao, and N. Chi, "A transform domain processing based channel estimation method for OFDM radio-over-fiber systems," in Proc. Opt. Fiber Commun. Conf., March 2013, paper OTu3D.2. [7] P.M. Anandarajah, R. Maher, Y. Q. Xu, S. Latkowski, J. O'Carroll, S.G. Murdoch, R. Phelan, J. O'Gorman, and L.P. Barry, "Generation of Coherent Multicarrier Signals by Gain Switching of Discrete Mode Lasers," IEEE Photon. J., vol.3, no.1, pp.112-122, Feb. 2011. [8] C.C Wei, C.T Lin, M. Chao, and W.J Jiang, "Adaptively Modulated OFDM RoF Signals at 60 GHz Over Long-Reach 100-km Transmission Systems Employing Phase Noise Suppression," IEEE Photon. Technol. Lett., vol.24, no.1, pp.49-51, Jan. 2012. [9] C.T Lin, C.C Wei, and M. Chao, "Phase noise suppression of optical OFDM signals in 60-GHz RoF transmission system," Opt. Express vol. 19, no. 11, pp. 10423-10428, May. 2011. [10] T. Shao, M. Beltran, R. Zhou, P.M Anandarajah, R. Llorente, and L.P Barry, "60 GHz Radio over Fiber System Based on Gain-Switched Laser," J. Lightw. Technol, vol.PP, no.99, pp.1,1 [11] T. Shao, F. Parésys, G. Maury, Y. Le Guennec, and B. Cabon, "Investigation on the Phase Noise and EVM of Digitally Modulated Millimeter Wave Signal in WDM Optical Heterodyning System," J. Lightw. Technol., vol.30, no.6, pp.876-885, March, 2012. [12] 100G CI-BCH-3 eFEC Encoder/Decoder Core and Design Package [Online] http://www.vitesse.com. [1]

(a) BTB

(b) 50km without compensation

(c) 50km with compensation Fig. 6. Constellations for 16QAM-OFDM 59.1 GHz signal over BTB fiber or 50 km SSMF with and without time delay compensation. Table I. EVM and BER of the 16QAM OFDM signals shown in Fig. 6. (a) (b) (c) BER 7.8E-4 3.7E-2 8.3E-4 EVM 14.62% 30.5% 14.6%

Fig. 7 (a) shows the EVM for the transmission of a 16QAM-OFDM 60-GHz signal at 25 Gb/s as a function of the received optical power when length compensation is used to ensure that the phase noise on the mmW signal is minimized. EVM values as low as 14.6% were achieved at the received optical power of -14 dBm. The BER as a function of received optical power is shown in Fig. 7 (b). As 7% of the signal is reserved for forward error correction (FEC), the receiver sensitivity is defined as the lowest optical power to ensure a BER lower than the FEC limit of 4.4×10-3 [12]. From Fig. 7 (b), it can be seen that optical receiver sensitivity is -26 dBm. There is no power penalty induced by the 50 km SSMF. On the contrary, the EVM and BER results with 50 km SSMF transmission are slightly better than those taken in the BTB case. This is because results in this case may not have been taken with perfect optical length matching between the two DEMUX channels.