Abstract-Phase estimation is one of the enabling functionalities in coherent optical orthogonal frequency-division-multiplexing (CO-OFDM) receivers. In this letter, we compare pilot-aided and data-aided phase estimation methods for a CO-OFDM transmission experiment at 8 Gb/s over 1000-km standard single-mode fiber without optical dispersion compensation. We also show that as few as five subcarriers are sufficient for pilot-aided phase estimation. IndexTerms-Coherent communications, orthogonal frequency-division multiplexing (OFDM), phase estimation.

In this paper, we conduct theoretical and experimental study on the PMD-supported transmission with coherent optical orthogonal frequency-division multiplexing (CO-OFDM). We first present the model for the optical fiber communication channel in the presence of the polarization effects. It shows that the optical fiber channel model can be treated as a special kind of multiple-input multiple-output (MIMO) model, namely, a two-input two-output (TITO) model which is intrinsically represented by a two-element Jones vector familiar to the optical communications community. The detailed discussions on various coherent optical MIMO-OFDM (CO-MIMO-OFDM) models are presented. Furthermore, we show the first experiment of polarization-diversity detection in CO-OFDM systems. In particular, a CO-OFDM signal at 10.7 Gb/s is successfully recovered after 900 ps differential-group-delay (DGD) and 1000-km transmission through SSMF fiber without optical dispersion compensation. The transmission experiment with higher-order PMD further confirms the immunity of the CO-OFDM signal to PMD in the transmission fiber. The nonlinearity performance of PMD-supported transmission is also reported. For the first time, nonlinear phase noise mitigation based on receiver digital signal processing is experimentally demonstrated for CO-OFDM transmission.

The first proof-of-concept experimental demonstration of coherent optical OFDM systems is reported. 128 OFDM subcarriers with a nominal data-rate of 8 Gbit=s are successfully processed and recovered after 1000 km transmission through SSMF fibre without optical dispersion compensation.

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Optical performance monitoring is an indispensable feature for optical systems and networks. In this paper, we propose the concept of optical performance monitoring through channel estimation by receiver signal processing. We show that in coherent-optical-orthogonal-frequency-division- multiplexed (CO-OFDM) systems, critical optical system parameters including fiber chromatic dispersion, Q value, and optical signal-to-noise ratio (OSNR) can be accurately monitored without resorting to separate monitoring devices.

Optical spectral measurements are crucial for optical sensors and many other applications, but the prevailing methods, such as optical spectrum analysis and tunable laser spectroscopy, often have to make compromises among resolution, speed, and accuracy. Optical frequency combs are widely used for metrology of discrete atomic and molecular spectral lines. However, they are usually generated by optical methods and have large comb spacing, which limits the resolution for direct sampling of continuous spectra. To overcome these problems, this paper presents an original method to digitally generate an ultrafine optical frequency comb (UFOFC) as the frequency ruler for spectral measurements. Each comb line provides one sampling point, and the full spectrum can be captured at the same time using coherent detection. For an experimental demonstration, we adopted the inverse fast Fourier transform to generate a UFOFC with a comb spacing of 1.46 MHz over a 10-GHz range and demonstrated its functions using a Mach-Zehnder refractive index sensor. The UFOFC obtains a spectral resolution of 0.01 pm and response time of 0.7 ms; both represent 100-fold improvements over the state of the art and could be further enhanced by several orders of magnitude. The UFOFC presented here could facilitate new label-free sensor applications that require both high resolution and fast speed, such as measuring binding kinetics and single-molecule dynamics.

Digital coherent superposition (DCS) provides an approach to combat fiber nonlinearities by trading off the spectrum efficiency. In analogy, we extend the concept of DCS to the optical OFDM subcarrier pairs with Hermitian symmetry to combat the linear and nonlinear phase noise. At the transmitter, we simply use a real-valued OFDM signal to drive a Mach-Zehnder (MZ) intensity modulator biased at the null point and the so-generated OFDM signal is Hermitian in the frequency domain. At receiver, after the conventional OFDM signal processing, we conduct DCS of the optical OFDM subcarrier pairs, which requires only conjugation and summation. We show that the inter-carrier-interference (ICI) due to phase noise can be reduced because of the Hermitain symmetry. In a simulation, this method improves the tolerance to the laser phase noise. In a nonlinear WDM transmission experiment, this method also achieves better performance under the influence of cross phase modulation (XPM).

Coherent optical OFDM systems employ DAC at the transmitters and ADC at the receivers. The sample frequencies of DAC and ADC in such systems need to be synchronized, especially in the context of high-speed transmissions. This paper presents a channel model including the effect of the sample frequency offset, which adds an additional phase shift proportional to the subcarrier index. The sample frequency offset monitoring and the compensation method are discussed and verified in experiment. It is expected that the synchronization can be achieved by feeding the monitoring result back to the receiver oscillator.