Experimental Demonstration of a Coherently Modulated and Directly Detected Optical OFDM System Using an RF-Tone Insertion

Abstract

We experimentally demonstrate an RF-tone assisted OFDM transmission. Our system has a 5-dB better sensitivity compared to the conventional baseband SSB-OFDM and exhibits a negligible penalty transmission after 260 km of SSMF. ◎2008 Optical Society of America OCIS codes: (060.2330) Fiber optics communications; (060.4080) Modulation IntroductionOptical orthogonal-frequency-division-multiplexing (OFDM) has recently received much attention due to its potential for electrical equalization of various deleterious effects, such as chromatic dispersion (CD) [1] and polarization-mode-dispersion (PMD) [2]. In general, coherent optical OFDM exhibits better sensitivity than an incoherent system. However, the coherent approach requires a local oscillator, polarization stabilizer, and phase estimation processing at the receiver.Alternatively, incoherent optical OFDM (IO-OFDM) combined with single-sideband transmission has been reported that uses a dual-drive Mach-Zehnder modulator (DD-MZM) [3][4][5]. However, that technique had a design trade-off between better sensitivity (i.e., high modulation depth) and robustness to chromatic dispersion (i.e., from nonlinearity in the modulator) [3,4]. In general, it would be quite advantageous to have the similar sensitivity and the transmission characteristics of a coherent system combined with the simple receiver design of an incoherent system.In this paper, we propose and experimentally demonstrate a coherently modulated and directly detected optical OFDM system using an inserted RF tone. We place one RF tone at the edge subcarrier for remote signal extraction after its beating with the data subcarriers. Thus, the RF tone enables coherent modulation at the transmitter and direct detection at the receiver. The optimum optical carrier to signal power ratio (CSPR) for the best sensitivity can be easily achieved by controlling the relative amplitudes of the electrical input signal. We demonstrate the RF tone assisted OFDM for an 8-QAM format at 10 Gbps. The experimental results show that the sensitivity of the proposed RF tone OFDM is 5 dB better than the conventional baseband SSB OFDM approach. After 260 km of standard single mode fiber (SSMF) transmission, there is negligible power penalty observed. Figure 1(a) shows the principle of the conventional baseband SSB-OFDM [3][4][5]. The data and its conjugate are transferred into a time domain real-valued electrical signal by inverse fast Fourier transform (IFFT). This realvalued electrical signal and its Hilbert transform are fed into the two arms of a quadrature biased DD-MZM resulting in a SSB format. Because the signal is intensity modulated, a photodiode can transfer the optical power into photocurrent directly. To evaluate the modulation efficiency, as shown in Fig. 1(c), we define the optical modulation index (OMI) as OMI V /V π where V is the root mean square of the electrical input to the DD-MZM and V is the switching voltage of the MZM. Figure 1(b) shows the proposed RF-tone assisted OFDM system with two possible subcarrier allocati...

Advanced modulation formats for high-performance short-reach optical interconnects

“…As shown in Fig. 2(b), RF tone assisted SSB signal reaches electrical SE of 1/2 at the transmitter when using (i) guard band, or (ii) subcarrier-interleaving [8,9] (see insets (i) and (ii) in Fig. 2(b)), and reaches electrical SE of 100% if no guard band is used (see inset (iii) in Fig.…”

“…In that way, the offset SSB signal takes 1/4 spectrum of the baseband, leading to the electrical SE of 1/4 at both transmitter and receiver. The electrical SE at the transmitter can be improved by using RF tone assisted SSB signal, where the RF tone introduces a virtual carrier at the frequency ω≠0 [8,9]. As shown in Fig.…”

“…For BPS scheme, full baseband is utilized to transmit the phase-diverse signal. Since the information is repeated in in two consecutive blocks, 1/2 electrical SE is achieved by BPS scheme, which is the same as the gapless SSB-DD [8]. By using the first approach in Fig.…”

“…Furthermore, the phase and polarization dimensions are not utilized by DM/DD, leading to low electrical spectrum efficiency (SE). Aiming to improve the electrical SE and extend the transmission distance, advanced DD modulation formats have been proposed through a so-called selfcoherent (SCOH) approach, where a carrier is transmitted together with the signal to achieve a linear mapping between the electrical signal and the optical field [7][8][9][10][11]. In that way, the impact of the CD can be easily removed from the received signal, greatly extending the transmission distance of the DD system.…”

“…One of the widely investigated SCOH schemes is single sideband (SSB) modulation format, where the signal is filled at one side of the carrier, enabling the recovery of the complex signal using a single-ended photodiode (PD) [7][8][9]. To avoid the interference of the signal-to-signal beat noise (SSBN) produced at the square-law detection, guard band or subcarrier interleaving is adopted, leading to the reduced electrical SE [7,8]. Gapless SSB modulation format is then proposed to improve the electrical SE by removing the guard band and employing iterative SSBN cancellation [9].…”