Spectrally efficient optical orthogonal frequency division multiplexing.

This paper charts the development of spectrally efficient forms of optical orthogonal frequency division multiplexing (OFDM) that are suited for intensity-modulated direct detection systems, such as wireless optical communications. The journey begins with systems using a DC-bias to ensure that no parts of the signal that modulates the optical source are negative in value, as negative optical intensity is unphysical. As the DC-part of the optical signal carries no information, it is wasteful in energy; thus asymmetrically clipped optical OFDM was developed, removing any negative-going peaks below the mean. Unfortunately, the clipping causes second-order distortion and intermodulation, so some subcarriers appear to be unusable, halving spectral efficiency; this is similar for unipolar and flipped optical OFDM. Thus, a considerable effort has been made to regain spectral efficiency, using layered techniques where the clipping distortion is mostly cancelled at the receiver, from a knowledge of one unpolluted layer, enabling one or more extra 'layers/paths/depths' to be received on the previously unusable subcarriers. Importantly, for a given optical power and high-order modulation, layered methods offer the best spectral efficiencies and need the lowest signal-to-noise ratios, especially if diversity combining is used. Thus, they could be important for high-bandwidth optical fibre systems. Efficient methods of generating all layers simultaneously, using fast Fourier transforms with their partial calculations extracted, are discussed, as are experimental demonstrations in both wireless and short-haul communications links. A musical analogy is also provided, which may point to how orchestral and rock music is deciphered in the brain. This article is part of the theme issue 'Optical wireless communication'.

Optical sampling to enhance Nyquist-shaped signal detection under limited receiver bandwidth.

Insufficient receiver bandwidth destroys the orthogonality of Nyquist-shaped pulses, generating inter-symbol interference (ISI). We propose using an optical pre-sampler to alleviate the requirement on the receiver bandwidth through pulse re-shaping. Experiments and simulations using an optically shaped 40-Gbaud Nyquist-shaped on-off-keying signal (N-OOK) show receiver sensitivity improvements of 4- and 7.1-dB under 18- and 11-GHz receiver electrical bandwidths, respectively.

All-optical OFDM demultiplexing with optical partial Fourier transform and coherent sampling.

We propose a novel scheme with a "time-lens"-based partial optical Fourier transform (OFT) and coherent sampling for high-speed complex orthogonal frequency-division multiplexing (OFDM) signal detection. Compared with all-optical OFDM demultiplexing with a matched optical filter, our proposed method replaces specialized optical filters with commercially available equipment, which relaxes stringent manufacturing and operational requirements. Our simulation shows that even with a partial OFT, theoretically, close to inter-channel interference-free performance is possible. In addition, we performed a proof-of-concept experiment of 16×10 Gbaud quadrature phase-shift keying (QPSK) all-optical OFDM detection, with all the bit error rates far below the 7% hard-overhead forward error correction limit.

All-optical digital-to-analog converter based on cross-phase modulation with temporal integration.

We propose and experimentally demonstrate an all-optical digital-to-analog converter based on cross-phase modulation with temporal integration. The scheme is robust for driving signal noise due to the low-pass filtering feature of the temporal integrator. The proof-of-concept experiment demonstrates the generation of pulse-amplitude modulation (PAM) sequences up to eight levels. The performance of random PAM 2 and PAM 4 signals with different optical signal-to-noise ratios of the binary driving signal is also investigated. The scheme is scalable for high-speed operation with an appropriate dispersion profile of the nonlinear medium.

Cycle-slip-less low-complexity phase recovery algorithm for coherent optical receivers.

We propose and experimentally validate a blind phase recovery algorithm based on tracking low-frequency components of the phase noise, which we call "filtered carrier-phase estimation (F-CPE)." Tracking only the low-frequency components allows F-CPE to reduce the computational complexity by using a frequency-domain equalizer and to simplify the partitioning of a 16 quadrature amplitude modulation (16QAM) constellation. Further, this approach eliminates cycle slips by suppressing the impact of amplified spontaneous emission on phase noise estimation. The experimental results demonstrate cycle-slip-free operation for 15 and 32 GBd 16QAM signals. Additionally, the proposed method showed similar or better sensitivity compared with the blind-phase-search algorithm, near standard forward error correction thresholds of modern wavelength division multiplexing systems.

Silicon microring modulator-based RF mixer for millimeter-wave phase-coded signal generation.

Phase-coded radio frequency (RF) pulses are widely adopted for radar systems as an effective signal format to enable high-range resolution. However, generating such signals conventionally requires high-speed electronics and complex RF circuitry that impose burdens on the system cost and power consumption. In particular, modern radar systems desire features such as high frequencies, e.g., in the millimeter-wave region, high compactness, and high system flexibility, which pose great challenges for the conventional all-electronics solutions. In contrast, integrated microwave photonics opens a way to solutions that are able to provide those features simultaneously, together with potential for full integration and low cost fabrication. Here, we present an integrated microwave photonic method of a binary-phase-coded millimeter-wave signal generation. The core device is a silicon microring modulator with a device size of 0.13 mm×0.32 mm and a modulation bandwidth of 23 GHz. Using RF seed frequencies of 17.5 GHz and 20 GHz, respectively, we experimentally demonstrated the generation of binary-phase-coded signals at 35 GHz and 40 GHz using our proposed approach, the performance of which was verified by a pulse compression ratio of 94 and 106, respectively. The result of this work points to the realization of a chip-scale flexible millimeter-wave signal generator.

Nyquist pulse shaping using arrayed waveguide grating routers.

We propose and demonstrate by simulations a novel Nyquist-WDM (N-WDM) superchannel transmitter based on an arrayed waveguide grating router (AWGR). This approach can generate Nyquist pulses at multiple wavelengths using a single AWGR. Results for a 3-channel 960-Gbit/s QPSK superchannel system show that a 10% guard band reduces the inter-channel interference (ICI) sufficiently. The design introduces less than 0.16-dB penalty when the waveguide loss is 2 dB/cm and 0.73-dB penalty when the standard deviation of phase error is 10°. Such Nyquist pulse shapers can be realised on a chip scale using photonic integrated circuits technology, and could be compactly integrated with other functional components to create single-chip N-WDM superchannel transmitters.

Banded all-optical OFDM super-channels with low-bandwidth receivers.

We propose a banded all-optical orthogonal frequency division multiplexing (AO-OFDM) transmission system based on synthesising a number of truncated sinc-shaped subcarriers for each sub-band. This approach enables sub-band by sub-band reception and therefore each receiver's electrical bandwidth can be significantly reduced compared with a conventional AO-OFDM system. As a proof-of-concept experiment, we synthesise 6 × 10-Gbaud subcarriers in both conventional and banded AO-OFDM systems. With a limited receiver electrical bandwidth, the experimental banded AO-OFDM system shows 2-dB optical signal to noise ratio (OSNR) benefit over conventional AO-OFDM at the 7%-overhead forward error correction (FEC) threshold. After transmission over 800-km of single-mode fiber, ≈3-dB improvement in Q-factor can be achieved at the optimal launch power at a cost of increasing the spectral width by 14%.

Photonic integrated circuit as a picosecond pulse timing discriminator.

We report the first experimental demonstration of a compact on-chip optical pulse timing discriminator that is able to provide an output voltage proportional to the relative timing of two 60-ps input pulses on separate paths. The output voltage is intrinsically low-pass-filtered, so the discriminator forms an interface between high-speed optics and low-speed electronics. Potential applications include timing synchronization of multiple pulse trains as a precursor for optical time-division multiplexing, and compact rangefinders with millimeter dimensions.

Comparisons of spectrally-enhanced asymmetrically-clipped optical OFDM systems.

Asymmetrically clipped optical orthogonal frequency-division multiplexing (ACO-OFDM) is a technique that sacrifices spectral efficiency in order to transmit an orthogonally frequency-division multiplexed signal over a unipolar channel, such as a directly modulated direct-detection fiber or free-space channel. Several methods have been proposed to regain this spectral efficiency, including: asymmetrically clipped DC-biased optical OFDM (ADO-OFDM), enhanced U-OFDM (EU-OFDM), spectral and energy efficient OFDM (SEE-OFDM), Hybrid-ACO-OFDM and Layered-ACO-OFDM. This paper presents simulations up to high-order constellation sizes to show that Layered-ACO-OFDM offers the highest receiver sensitivity for a given optical power at spectral efficiencies above 3 bit/s/Hz. For comparison purposes, white Gaussian noise is added at the receiver, component nonlinearities are not considered, and the fiber is considered to be linear and dispersion-less. The simulations show that LACO-OFDM has a 7-dB sensitivity advantage over DC-biased OFDM (DCO-OFDM) for 1024-QAM at 87.5% of DCO-OFDM's spectral efficiency, at the same bit rate and optical power. This is approximately equivalent to a 4.4-dB advantage at the same spectral efficiency of 87.7% if 896-QAM were to be used for DCO-OFDM.

Time-lenses for time-division multiplexing of optical OFDM channels.

Time-lenses provide a promising platform for novel, broadband optical signal processing. However, in order to minimize system penalties, design constraints must be adequately taken into account. We investigate the impact of third-order-dispersion and nonlinear distortion on the performance of time-lens-based communication systems for the first time. Here, we propose a novel application of time-lenses - temporal compression and time-division multiplexing of optical OFDM channels, to provide a 1 Tb/s superchannel. Time-lens system performance degradations are investigated in our proposed system and the results are applicable to all four wave mixing based time-lens systems. Our work can help to optimize time-lens based communication systems.

Improved polarization dependent loss tolerance for polarization multiplexed coherent optical systems by polarization pairwise coding.

Polarization dependent loss (PDL) causes imbalanced optical signal to noise ratio (OSNR) of the two polarizations, thus remains one of the major bottlenecks for next-generation polarization-division-multiplexed (PDM) coherent optical transmission systems. In this paper, we investigate Pairwise Coding for adaptive PDL mitigation in PDM coherent optical systems. By pre-coding across two polarizations, the PDL-induced performance degradation can be largely mitigated without any coding overhead. We present details of the coding and de-coding design, and also derive the analytical symbol/bit error rate of the Polarization Pairwise Coding scheme, which can be used to predict the performance gain as well as for optimal rotation angle calculation. Simulation results verify that Pairwise Coding achieves substantial system performance gains over a wide range of PDL values. Compared with other digital coding techniques, Polarization Pairwise Coding shows improved performance than Walsh-Hadamard transform since it maximizes the coordinate diversity; and also Pairwise Coding is computationally much simpler to decode compared with the Golden and Silver Codes, therefore is practical for current 100-Gb/s and future 400-Gb/s and 1-Tb/s digital coherent transceivers.

Widely-tunable low-phase-noise coherent receiver using an optical Wadley loop.

The Wadley Loop is a method of down-converting RF signals over a wide frequency range using a low-quality widely-tunable oscillator and a high-stability frequency comb reference. Together the widely tunable oscillator and high-stability comb source provide a widely-tunable high-stability receiver. In this paper, we demonstrate an electro-optic version of the Wadley Loop that is able to provide a widely-tunable, high phase stability coherent receiver. This could have applications in Quadrature Amplitude Modulation (QAM) receivers with high constellation sizes, optical OFDM receivers with long symbol durations, and wide-range high spectral resolution optical spectrum analysers.

Systems performance comparison of three all-optical generation schemes for quasi-Nyquist WDM.

Orthogonal time division multiplexing (OrthTDM) interleaves sinc-shaped pulses to form a high baud-rate signal, with a rectangular spectrum suitable for multiplexing into a Nyquist WDM (N-WDM)-like signal. The problem with generating sinc-shaped pulses is that they theoretically have infinite durations, and even if time bounded for practical implementation, they still require a filter with a long impulse response, hence a large physical size. Previously a method of creating chirped-orthogonal frequency division multiplexing (OFDM) pulses with a chirped arrayed waveguide (AWG) filter, then converting them into interleaved quasi-sinc pulses using dispersive fiber (DF), has been proposed. This produces a signal with a wider spectrum than the equivalent N-WDM signal. We show that a modification to the scheme enables the spectral extent to be reduced for the same data rate. We then analyse the key factors in designing an OrthTDM transmitter, and relate these to the performance of a N-WDM system. We show that the modified transmitter reduces the required guard band between the N-WDM channels. We also simulate a simpler scheme using an unchirped finite-impulse response filter of similar size, which directly creates truncated-sinc pulses without needing a DF. This gives better system performance than either chirped scheme.

All-optical DAC using counter-propagating optical and electrical pulses in a Mach-Zehnder modulator.

A novel method of converting binary-level electrical pulses into multi-level optical pulses using only a conventional traveling-wave optical modulator is presented. The method provides low inter-pulse interference due to the counter-propagating pulses, low amplitude noise, and a timing jitter determined chiefly by the quality of the optical pulse source. The method only requires one electrical drive per modulator and provides low-jitter variable-amplitude optical pulses that are suitable for shaping into a wide variety of modulation formats using a programmable optical filter.

All-optical generation of DFT-S-OFDM superchannels using periodic sinc pulses.

Discrete-Fourier-transform spread (DFT-S) optical Orthogonal Frequency Division Multiplexed (OFDM) signals offer improved nonlinearity performance in long haul optical communications systems, and can be used to form superchannels. In this paper we propose how DFT-S-OFDM superchannels can be generated and demultiplexed using all-optical techniques, and demonstrate the feasibility using numerical simulations. We also discuss how each wavelength channel is similar to recently proposed Orthogonally Time-Division Multiplexed (OrthTDM) systems using periodic-sinc pulses from, for example, a Nyquist laser. The key difference between OrthTDM and DFT-S-OFDM is the synchronization of the symbol boundaries of every modulation tributary; because of this we show that OrthTDM cannot be formed into superchannels that can be demultiplexed without penalties, but DFT-S-OFDM can be.

Flexible all-optical frequency allocation of OFDM subcarriers.

We investigate the underlying mechanism that allows OFDM subcarriers in an all-optical OFDM system to be assigned to any optical frequency using an optical filter, even if that frequency is not generated by the comb-line source feeding the filters. We confirm our analysis using simulations, and present experimental results from a 252-subcarrier system that uses a mode-locked laser (MLL) as the comb source and a wavelength selective switch. The experimental results show that there is no correlation between the programmed frequency offset between a subcarrier and nearest comb line, and the received signal quality. Thus, subcarriers could be inserted into unused portions of an optical transmission system's spectrum without restriction on their particular center frequencies. Any percentage of cyclic prefix can be added to the OFDM symbol simply by reprogramming the optical filter to give wider subcarrier frequency spacing than the comb line spacing, which is useful for tailoring the CP to the dispersion of various optical transmission paths, to maximize the spectral efficiency. Finally, the MLL's center frequency need not be locked to a system reference.

Inserting a cyclic prefix using Arrayed-Waveguide Grating Routers in all-optical OFDM transmitters.

Arrayed-Waveguide Grating Routers (AWGR) can be used as multiplexers and demultiplexers in optical OFDM systems, as they provide both the serial-to-parallel converter and the optical Fourier transform in one component. This paper shows how the design of the AWGR at the transmitter can be modified to insert a cyclic prefix or postfix (CP). We use simulations of a 4-subcarrier system to compare systems without the CP, with a guard-interval, and with a CP. We show that the CP greatly improves the orthogonality of the subcarriers and resilience to timing errors. Furthermore, the CP allows for uncompensated fiber dispersion, especially if the relative timing of the subcarriers upon transmission is adjusted.

All-optical OFDM transmitter design using AWGRs and low-bandwidth modulators.

An Arrayed-Waveguide Grating Router (AWGR) can be used as a demultiplexer for an optical OFDM system, as it provides both the serial-to-parallel converter and the optical Fourier transform (FT) in one component. Because an inverse FT is topologically identical to a Fourier transform, the AWGR can also be used as a FT in an OFDM transmitter. In most all-optical OFDM systems the optical modulators are fed with CW tones; however, the subcarriers (SC) will only be perfectly orthogonal if the bandwidth of the data modulators is similar to the total bandwidth of all subcarriers. Using simulations, this paper investigates the reduction in modulator bandwidth that could be achieved if the modulators are placed before an AWGR designed as a FT. This arrangement also allows the complex (IQ) modulators to be replaced with simpler and more-compact phase modulators. We show that these design improvements enable 7.5-GHz bandwidth modulators to be used in a 4 × 10 Gsymbol/s (80 Gbit/s) per polarization per wavelength system.

Experimental demonstration of a flexible and stable semiconductor laser linewidth emulator.

We propose and demonstrate experimentally a laser source whose linewidth is adjustable independently of its other characteristics. This source can be used to test whether a particular laser would be suitable in a system, without the need to purchase several different lasers. It also has the advantage that the linewidth is generated digitally so it is extremely stable over time. We demonstrate a dialed-linewidth emulator between 256 kHz to 150 MHz. The narrowest linewidth shown by this technique is the original linewidth of the semiconductor laser source used in the setup. We also investigate the effect of driving our modulator into its nonlinear range.