Coding for stable transmission of W-band radio-over-fiber system using direct-beating of two independent lasers

L. G. Yang, J. Y. Sung, C. W. Chow, C. H. Yeh, K. T. Cheng, J. W. Shi, and C. L. Pan Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan Department of Photonics, Feng Chia University, Seatwen, Taichung 40724, Taiwan Department of Physics and Institute of Photonics Technologies, National Tsing-Hua University, Hsinchu, Taiwan Department of Electrical Engineering, National Central University, Taoyuan, Taiwan *cwchow@faculty.nctu.edu.tw


Introduction
Millimeter-wave (mm-wave) communications at W-band (75 GHz -110 GHz) using radioover-fiber (ROF) technology have attracted attenuations recently [1][2][3][4].For the ROF systems, an optical comb [1,4] is usually required to produce optical signal for launching into a high speed photodiode (PD) to generate mm-wave signals, in which the wavelength separation between the optical comb lines is equal to the frequency of the mm-wave carrier produced.The optical comb can be generated using a mode-locked pulse laser [1,5], electro-optical schemes [2,6,7], or the direct-beating between two independent continuous-wave (CW) lasers [3].Among these methods, the direct-beating between two lasers is attractive since the frequency separation of the optical comb lines is not limited by the electrical source.Hence sub-terahertz or terahertz signals can be easily generated and the expensive mode-locked pulse laser is not required.Expensive CW laser source uses external feedback loop (FL), such as frequency tracking circuit (FTC) [8,9] to maintain the output wavelength stability.For the cost sensitive customer end, low-cost CW laser sources without FTC, such as Fabry-Perot laser diode (FP-LD) [10,11] are used.However, when using two low-cost CW lasers as localcarrier source in the direct-beating approach, this introduces signal instability in the opticalbeating-interference sub-terahertz signal (OBIS) produced by the PD.The signal instability in the OBIS is mainly caused the differences in polarizations and cavity lengths between the two independent CW lasers.The accumulated beating noises, named optical-beating-interference noises (OBIN) are measured.The dominant OBIN is the low-frequency-components (LFCs) beating interference, which are at 10-240 MHz.Such LFCs make a low-frequency modulation to the intermediate frequency (IF) data and reduce the signal-to-noise ratio (SNR).
In this work, we demonstrate experimentally Manchester (MC) coding based W-band ROF system to reduce the LFCs signal distortion by using spectral shaping.Hence, a low-cost and higher performance W-band ROF system is achieved.The error-free transmission (biterror rate (BER)<10 −12 ) using MC coding is achieved at photocurrent of 5.5 mA and at DC external bias of 1.8 V.It requires 1.6 mA less photocurrent when compared with using nonreturn-to-zero on-off-keying (NRZ-OOK) signal.The improvement of PD sensitivity is due to the use of MC coding, which exhibits higher tolerance to low-frequency noises, As a result, for the same BER performance, less optical signal power is needed in the MC coding signal than that in NRZ-OOK signal.In this system, direct-beating of two independent low-cost CW lasers without FTC serve as the optical local-carrier (O-LO) to generate the mm-wave.Approaches, such as delayed self-heterodyne interferometer and heterodyne beating are performed to characterize the OBIS.Furthermore, W-band ROF systems using MC coding and NRZ-OOK are compared and discussed.

OBIS, OBIN, and LFCs of two independent low-cost CW lasers
At transmitter (Tx) of our ROF system, an data signal is frequency up-converted by O-LO, which is produced by two independently commercial DFB lasers.Although in the experiment, the wavelength separation of the two lasers are 0.8 nm (~100 GHz), in order to fit the measuring bandwidth of the electrical spectrum analyzer (ESA, Agilent E4440A), the wavelength separation are at 0.08 nm.Hence, the direct-beating by the two lasers (i.e. the OBIS) corresponds to a 10 GHz tone on electrical spectrum.The setup is shown in Fig. 1(a).It consists of the two CW lasers, a 50:50 optical couplers (CP), two polarization controllers (PCs), a 10 GHz photodiode (PD, Multiplex Inc. MTRX192L).The frequency instability of the 10 GHz tone is shown in Fig. 1(b).The different colors represent the same OBIS which are tracked at different time.The fluctuation over time is within in a range of 240 MHz.It is worth to mention that no FTC is used in our lasers.In the literatures, a stable beating signal can be achieved by optical phase-locked loop [12] or FTC in tunable laser source [13].At receiver (Rx) of our ROF system, the OBIS is detected and down-converted by envelope detection.The envelope-detection system can be simpler and less expensive than the coherent-detection system, and no high-speed electrical local oscillators and mixers are needed.The envelope-detection is considered as one of the promising candidates for mmwave systems [13,14].The accumulated beating noises, named OBIN introduce lowfrequency modulation to the IF data and reduce the ROF performance.The instability of the two CWs is carried out in following.First, to characterize the dependence of longitudinal modes on polarization state of single CW laser, a delayed self-heterodyne interferometer method was utilized [15].The setup is shown in Fig. 1(c).It consists of a CW laser, two 50:50 CPs, two PCs, an electrical amplifier (EA), a 10 GHz phase modulator (PM, EOSPACE LN53S-FC), a 10 GHz PD, and an ESA.The CW laser with optical power of 6 dBm is launched to a CP and split into two paths.One path is modulated by a 10 GHz sinusoidal signal via a PM and the other path is delayed by a 20 km standard single-mode fiber (SSMF).The delayed heterodyne detection with a 20 km delay length can avoid unwanted beating signals.Finally, OBIS of self-heterodyne is measured by the ESA.The measurement resolution of the method is given by Res = c/n d L d , where Res is the resolution of measurement, c is the speed of light and n d and L d are the refractive index and the length of the fiber delay line, respectively.The presented resolution is about 10 kHz.By Gaussian fitting the measurement results, the 3-dB linewidth of the CW laser was determined to be 73 MHz, as shown in Fig. 1(d).
To further evaluate the effects on OBIN, including unequal polarization state and cavity length of the two independent lasers, a heterodyne beating with a sinusoidal signal is performed for single-sideband (SSB) power spectral density (PSD) measurement, as shown in Fig. 2(a).This measurement is similar to that shown in Fig. 1(c), instead of using a single laser, the two lasers are set at the same wavelength of 1550 nm.The Mach-Zehnder modulator (MZM) is driven by a 2 GHz electrical sinusoidal signal.Figure 2(b) shows the measured SSB PSD, with the carrier frequency at 2 GHz.We can observe all the OBIN, and the dominant LFCs are from 10 MHz to 240 MHz.Then, the wavelength separation of two independent CW lasers is adjusted to 0.08 nm (10 GHz) as shown in Fig. 2(c), and we now measure the SSB PSD at the carrier frequency of 10 GHz (corresponds to the 0.08 nm wavelength separation of the two lasers).By zooming the 2 GHz tone (f IF produced by the modulation via the MZM) of the SSB PSD shown in Fig. 2(d), we can observe the 240 MHz modulation LFCs signal.Our NBUTC-PD [16] has an active area of 100 μm 2 .It is flip-chip mounted on a 100μmthick aluminum nitride (AlN) substrate for good thermal conductivity.Molecular beam epitaxy (MBE) is used to grow the NBUTC-PD on Indium Phosphide (InP).On the AlN substrate, there are also a co-planar waveguide, an intermediate-frequency (IF) input port, a W-band RF choke and a planar quasi-Yagi radiator.The quasi-Yagi radiator is designed so that it can be directly fed into a WR-10 waveguide W-band horn antenna.

NBUTC
The MC encoding and decoding schemes are briefly described here.Figures 4(a) and 4(b) illustrate the schematic bit-patterns of "1011" for the NRZ-OOK and the MC coded signal.As shown in Fig. 4(b), the signal transition from low to high represents logic "1", while the signal transition from high to low represents logic "0".At the Rx, the received MC signal is power divided into two parts.One part is half-bit delayed, as shown in Fig. 4(c).Finally, the received MC signal subtracts its half-bit delayed signal.After this delay-and-subtraction decoding process, the MC signal can be decoded as shown in Fig. 4(d).It can be seen that the original bit-pattern of "1011" can be decoded successfully.

Fig. 2 .
Fig. 2. (a) Experiment of a heterodyne beating of two individual lasers with same wavelength.(b) Measured SSB PSD at carrier frequency of 2 GHz.(c) Experiment of a heterodyne beating of two individual lasers with wavelength separation of 0.08 nm.(b) Measured SSB PSD at carrier frequency of 10 GHz (0.08 nm).

Figure 3
Figure 3 shows the experimental setup of the W-band ROF system.The Tx includes the O-LO, an IF generation part and a NBUTC-PD.The wavelength separation between the lasers is 0.8 nm (i.e. 100 GHz).Then the optical signals are combined via a 50:50 CP and then amplified by an erbium doped fiber amplifier (EDFA) before launching into the NBUTC-PD.The NRZ-OOK and the MC coded electrical data are generated by an arbitrary waveform generator (AWG, Tektronix AWG7083C) with sampling rate of 8 GSample/s and bandwidth of 3 GHz.The generated electrical data is amplified and then applied to the NBUTC-PD at the IF port via a bias-tee and an RF probe.DC bias of 1.8 V is used for both NRZ-OOK and MC coding experiments.The function of the NBUTC-PD is a RF mixer for frequency upconverting the IF data to the W-band mm-wave signal, which is then transmitted wirelessly by a W-band horn antenna (Quinstar, QWH-WPRR00).At the Rx, the W-band wireless signal is received by another W-band horn antenna and amplified by a low-noise amplifier (Millitech, LNA-10-02150).The signal is then envelope-detected by an envelope detector

Fig. 4 . 2 (
Fig. 4. Schematic bit-patterns of (a) NRZ-OOK, (b) MC signal, (c) half-bit delayed MC signal, and (d) decoded Manchester signal.Simulated power spectra of the (e) received NRZ-OOK signal, (f) received MC coded signal, (g) transfer function of decoding process and (h) decoded MC coded signal.Then, we analyze the delay-and-subtraction decoding property of the MC coding.It will be shown that this feature makes possible the removal of the LFCs.Since the LFCs cannot be easily extracted from insets (a) and (b) of Fig. 3 directly, a numerical analysis to illustrate the delay-and-subtraction decoding property of MC coding is performed.The MC decoding process can be represented by a transfer function in Eq. (1), ( ) ( ) ( ) h t t t δ δ τ = − − (1) where τ is the half-bit delay time in the decoding process.H(f) is the Fourier transform of h(t), can then be expressed in Eq. (2), 2 ( ) 1 exp j f H f π τ − = −(2) The power of the transfer function is then expressed in Eq. (3), in Eq. (3) does not have a DC (f = 0) component, while its value is a maximum at f = 1/2τ.The simulated power spectra of the NRZ-OOK and MC coding signals, X 1 (f) and X 2 (f), are shown in Figs.4(e) and 4(f), respectively.LFCs are added at frequencies from 0.1/τ to 0.2/τ.As the LFCs have high spectral overlapping with the NRZ-OOK signal, it