Carrierless amplitude and phase modulation in wireless visible light communication systems.

Visible light communications (VLCs) have attracted considerable interest in recent years owing to the potential to simultaneously achieve data transmission and illumination using low-cost light-emitting diodes (LEDs). However, the high-speed capability of such links is typically limited by the low bandwidth of LEDs. As a result, spectrally efficient advanced modulation formats have been considered for use in VLC links in order to mitigate this issue and enable higher data rates. Carrierless amplitude and phase (CAP) modulation is one such spectrally efficient scheme that has attracted significant interest in recent years owing to its good potential and practical implementation. In this paper, we introduce the basic features of CAP modulation and review its use in the context of indoor VLC systems. We describe some of its attributes and inherent limitations, present related advances aiming to improve its performance and potential and report on recent experimental demonstrations of LED-based VLC links employing CAP modulation. This article is part of the theme issue 'Optical wireless communication'.

Monolithically integrated tunable mode-locked laser diode source with individual pulse selection and post-amplification.

We report the generation of high-peak-power picosecond optical pulses in the 1.55 µm spectral band from a monolithically mode-locked laser integrated with a pulse selector and power booster. High-peak-power (>1 W) pulses with durations of 15.4 ps at a 55 MHz selected rate are demonstrated, indicating that this device shows promise as a high-peak-power pulsed light source for bio-photonic applications.

Quantum key distribution over multicore fiber.

We present the first quantum key distribution (QKD) experiment over multicore fiber. With space division multiplexing, we demonstrate that weak QKD signals can coexist with classical data signals launched at full power in a 53 km 7-core fiber, while showing negligible degradation in performance. Based on a characterization of intercore crosstalk, we perform additional simulations highlighting that classical data bandwidths beyond 1Tb/s can be supported with high speed QKD on the same fiber.

High speed OFDM-CDMA optical access network.

We demonstrate the feasibility of a 16 × 3.75 Gb/s (60 Gb/s aggregate) Orthogonal frequency division multiplexing-code division multiple access passive optical network for next-generation access applications. 3.75 Gb/s PON channel transmission over 25 km single-mode fiber shows 0.1 dB dispersion and 0.9 dB crosstalk penalties. Advantages of the system include high capacity, enhanced spectral efficiency, coding gain, and networking functions such as increased security and single-wavelength operation.

First demonstration of OFDM ECDMA for low cost optical access networks.

We demonstrate for the first time to the best of our knowledge an analogue orthogonal frequency division multiplexing (OFDM) based electrical code division multiplexing access (ECDMA) passive optical network (PON) for next generation access applications. Advantages of the system include low cost, high capacity, and enhanced spectral efficiency. A proof-of-principle 16 QAM OFDM ECDMA PON downlink experiment is used to show the transmission of an aggregate data rate of 24.8 Gb/s within an eight-user system. Transmission is achieved over 25 km of single-mode telecommunications fiber (SMF) with negligible dispersion and crosstalk penalties.

Experimental demonstration of optical data links using a hybrid CAP/QAM modulation scheme.

The first known experimental demonstrations of a 10 Gb/s hybrid CAP-2/QAM-2 and a 20 Gb/s hybrid CAP-4/QAM-4 transmitter/receiver-based optical data link are performed. Successful transmission over 4.3 km of standard single-mode fiber (SMF) is achieved, with a link power penalty ∼0.4 dBo for CAP-2/QAM-2 and ∼1.5 dBo for CAP-4/QAM-4 at BER=10(-9).

Monolithic MZI-SOA hybrid switch for low-power and low-penalty operation.

We report the first experimental demonstration of a monolithically integrated hybrid dilated 2×2 modular optical switch using Mach-Zehnder modulators as low-loss 1×2 switching elements and short semiconductor optical amplifiers to provide additional extinction and gain. An excellent 40 dB cross-talk/extinction ratio is recorded with data-modulated signal-to-noise ratios of up to 44 dB in a 0.1 nm bandwidth. A switching time of 3 ns is demonstrated. Bit error rate studies show extremely low subsystem penalties of less than 0.1 dB, and studies indicate that, by using this hybrid switch building block, an 8×8 port switch could be achieved with 14 dB input power dynamic range for subsystem penalties of less than 0.5 dB.

Demonstration of the feasibility of large-port-count optical switching using a hybrid Mach-Zehnder interferometer-semiconductor optical amplifier switch module in a recirculating loop.

For what we believe is the first time, the feasibility of large-port-count nanosecond-reconfiguration-time optical switches is demonstrated using a hybrid approach, where Mach-Zehnder interferometric (MZI) switches provide low-loss, high-speed routing with short semiconductor optical amplifiers (SOAs) being integrated to enhance extinction. By repeatedly passing signals through a monolithic hybrid dilated 2×2 switch module in a recirculating loop, the potential performance of high-port-count switches using the hybrid approach is demonstrated. Experimentally, a single pass switch penalty of only 0.1 dB is demonstrated for the 2×2 module, while even after seven passes through the switch, equivalent to a 128×128 router, a penalty of only 2.4 dB is recorded at a data rate of 10 Gb/s.

Efficient decoy-state quantum key distribution with quantified security.

We analyse the finite-size security of the efficient Bennett-Brassard 1984 protocol implemented with decoy states and apply the results to a gigahertz-clocked quantum key distribution system. Despite the enhanced security level, the obtained secure key rates are the highest reported so far at all fibre distances.

Gigabit NRZ, CAP and optical OFDM systems over POF links using LEDs.

Simulations have been performed to compare the system capacity and power dissipation of NRZ, CAP-64 and 64-QAM-OFDM systems over FEC enhanced POF links using LEDs, for both unidirectional and bidirectional transmission. It is shown that CAP-64 outperforms NRZ and 64-QAM-OFDM in terms of system capacity and supports a record high 3.5 Gb/s bidirectional and 2.1 Gb/s unidirectional transmissions over 50 m POF. The CAP-64 transceiver consumes similar power compared with NRZ whilst the 64-QAM-OFDM transceiver consumes about twice as much.

Regenerative polymeric bus architecture for board-level optical interconnects.

A scalable multi-channel optical regenerative bus architecture based on the use of polymer waveguides is presented for the first time. The architecture offers high-speed interconnection between electrical cards allowing regenerative bus extension with multiple segments and therefore connection of an arbitrary number of cards onto the bus. In a proof-of-principle demonstration, a 4-channel 3-card polymeric bus module is designed and fabricated on standard FR4 substrates. Low insertion losses (≤ -15 dB) and low crosstalk values (< -30 dB) are achieved for the fabricated samples while better than ± 6 µm -1 dB alignment tolerances are obtained. 10 Gb/s data communication with a bit-error-rate (BER) lower than 10(-12) is demonstrated for the first time between card interfaces on two different bus modules using a prototype 3R regenerator.

Algorithm-based continuous pulse duration tuning and performance control of a mode-locked laser diode.

A control algorithm is presented that addresses the stability issues inherent to the operation of monolithic mode-locked laser diodes. It enables a continuous pulse duration tuning without any onset of Q-switching instabilities. A demonstration of the algorithm performance is presented for two radically different laser diode geometries and continuous pulse duration tuning between 0.5 ps to 2.2 ps and 1.2 ps to 10.2 ps is achieved. With practical applications in mind, this algorithm also facilitates control over performance parameters such as output power and wavelength during pulse duration tuning. The developed algorithm enables the user to harness the operational flexibility from such a laser with 'push-button' simplicity.

Ultrashort superradiant pulse generation from a GaN/InGaN heterostructure.

Dicke superradiance from a two-section violet GaN/InGaN semiconductor laser diode is demonstrated for the first time. In the superradiance regime, optical pulses with peak powers in excess of 2.8 W and durations as short as 1.4 ps are generated at repetition rates of up to 10 MHz at the emission wavelength of 408 nm. The properties of superradiant pulse generation from these GaN/InGaN laser diodes are very similar to those reported for infrared AlGaAs/GaAs laser diodes.

Spectrally efficient next-generation optical access network incorporating a novel CWDM uplink combiner.

A novel wavelength division multiplexed subcarrier multiplexed (WDM/SCM) broadband (1-Gb/s per user) optical access network (OAN) architecture incorporating a coarse WDM (CWDM) uplink combiner is proposed. The concept is demonstrated through theoretical and experimental validation of a 10 x1-Gb/s quadurature-phase-shift keying (QPSK) SCM optical network. Low penalty transmission is demonstrated for a proof-of-principle dual channel system with a range of 25 km. In agreement with simulation, experiments show that channel spacings of only 1 GHz are viable for Q factors of more than 18 dB. Simulations indicate that the system will operate with 40 wavelengths, each carrying 10 SCM channels at 1 Gb/s.

Single SOA based 16 DWDM channels all-optical NRZ-to-RZ format conversions with different duty cycles.

We demonstrate simultaneous NRZ-to-RZ conversion for 16 DWDM channels, using a single SOA and a subsequent delay interferometer (DI) acting as a comb-like filter to control the obtained pulse-width for all of the channels. The SOA is operated in deep saturation, resulting in weak cross gain modulation and cross phase modulation induced crosstalk between different NRZ channels. By adjusting the detuning between the peaks in the DI spectrum and each corresponding carrier, good quality RZ signals with different duty cycles can be achieved. Bit-error-rate measurements show negative power penalties for the obtained RZ signals with different duty cycles. Significant timing jitter reductions for all channels show the good regenerative performance of the proposed converter.

158-microJ pulses from a single-transverse-mode, large-mode-area erbium-doped fiber amplifier.

We report the amplification of 10-100-pJ semiconductor diode pulses to an energy of 158 microJ and peak powers >100 kW in a multistage fiber amplifier chain based on a single-mode, large-mode-area erbium-doped amplifier design. To our knowledge these results represent the highest single-mode pulse energy extracted from any doped-fiber system.