We demonstrate a linearly field-modulated, direct-detected virtual SSB-OFDM (VSSB-OFDM) transmission with an RF tone placed at the edge of the signal band. By employing the iterative estimation and cancellation technique for the signal-signal beat interference (SSBI) at the receiver, our approach alleviates the need of the frequency gap, which is typically reserved for isolating the SSBI, and saves half the electrical bandwidth, thus being very spectrally efficient. We derive the theoretical model for the VSSB-OFDM system and detail the signal processing for the iterative approach conducted at the receiver. Possible limitations for this iterative approach are also given and discussed. We successfully transmit a 10 Gbps, 4-quadrature-amplitude-modulation (QAM) VSSB-OFDM signal through 340 km of uncompensated standard single mode fiber (SSMF) with almost no penalty. In addition, the simulated results show that the proposed scheme has an approximately 2 dB optical-signal-to-noise-ratio (OSNR) gain and has a better chromatic dispersion (CD) tolerance compared with the previous intensity-modulated SSB-OFDM system.

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...

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We propose and experimentally demonstrate an optical signal-to-noise ratio (OSNR) monitoring technique for an 80 Gbits/s polarization-multiplexed return-to-zero differential phase-shift keying channel utilizing a narrowband optical filter and a low-speed detector. A maximum power increment of 19.7 dB is measured at a radio frequency (rf) of 250 MHz for monitoring of OSNR up to 27 dB, insensitive to chromatic dispersion of 0-300 ps/nm and polarization-mode dispersion (PMD) of 0-50 ps.