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A coherent interference‐free tunable microwave photonic notch filter with a pair of negative and positive coefficients generated by employing polarisation multiplexing and cross gain modulation in a semiconductor optical amplifier is proposed and demonstrated through simulation results. The negative and positive coefficients of the notch filter are realised through two orthogonal polarisation states of a single continuous wave laser diode. The free spectral range of microwave photonic filter is tuned by varying the differential group delay of a birefringent single‐mode fibre. The proposed filter shows good tunability, easy implementation and is cost‐effective. [ABSTRACT FROM AUTHOR]

A new coupling scheme of a 4.28-THz quantum-cascade laser is constructed by employing two off-axis parabolic reflectors. Two parallel terahertz beams are coupled out from the symmetrical facets of the wire laser. The total effective laser power of the terahertz beams is measured by a thermal power-meter under the modulation state. The highest value of the laser effective output power is about 1.4 W at a driving current of 19.3 A and a duty cycle of 1%. This dual-beam coupling scheme makes full use of the output power of the wire laser and has important applications in terahertz imaging and biological-effect research studies. [ABSTRACT FROM AUTHOR]

In this Letter, a recorded single channel of 6.915-Gbit/s white-light visible light communication (VLC) system is reported and experimentally demonstrated. By optimising the bit-loading algorithm onto the direct-current optical OFDM signal and without using an optical blue filter, the high data rate can be achieved. After a free-space propagation distance of 1.5 m, the white-light beam diameter of 14 cm and illuminance of 795 lux are measured. The proposed white-light VLC system can provide both lighting and communication simultaneously with functional propagation distance. [ABSTRACT FROM AUTHOR]

This Letter presents a novel RF ground lead structure for surface-mount-type optical modules that enables RF crosstalk reduction and bandwidth enhancements. Conventional RF lead-pins of a surface-mount-type optical module have a bent shape that affects on RF ground stability and RF crosstalk between channels. The authors introduced an enhanced ground lead with an optimised shape for RF ground scheme enhancement. They prepared two types of high-bandwidth coherent driver modulator packages for demonstrating RF performance difference, one with the conventional bent ground lead and the other with the optimum-shaped ground lead. The measured results of the package with the enhanced ground lead successfully exhibited adjacent far-end crosstalk reduction of about 5 dB at 50 GHz, and the crosstalk of −30 dB or less up to 58 GHz was confirmed. Moreover, they obtained the improvement in the insertion loss of 1 dB at 54 GHz and the 3-dB electrical bandwidth enhancement of 2 GHz. These RF performances are sufficient for a high-bandwidth coherent driver modulator package that allows 96-GBd class operations. [ABSTRACT FROM AUTHOR]

An integrated optical digital-to-analogue (DA) converter, which is intended for a phase-shift keying (PSK) modulation signal, is presented to realise a fully transparent optical communication system. The converter is provided with a silicon-based optical waveguide structure in a compact size equal to 1/3000 that of the previous study. The device optically performs a conversion of digital data to their equivalent analogue values by generating a quadrature PSK (QPSK) signal from two binary PSK (BPSK) modulated pulses. In the author's experiment, the resulting QPSK signal is converted into four different intensities depending on the pattern of the 2-bit BPSK signal at 10 GHz. This fact indicates the experimental demonstration of optical DA conversion for PSK modulation using the integrated device. [ABSTRACT FROM AUTHOR]

An optical frequency divider for dual-wavelength optical signals based on an optoelectronic oscillator (OEO) is proposed and experimentally demonstrated. The Mach–Zehnder modulator in the OEO is biased at the minimum transmission point, the OEO-based frequency divided signal originates from system noise. When the oscillation phase and amplitude conditions met, it gets stabilised at an oscillation frequency which is one-third of the frequency interval of the injected dual-wavelength optical signal. A proof-of-concept experiment is verified. With the frequency interval of the dual-wavelength optical signal ranging from 101.4 to 120.6 GHz, the RF signals ranging from 33.8 to 40.2 GHz are successfully generated. The power ripple is lower than 2.92 dB and the rejection ratio of the harmonic components is larger than 39 dB for all the conditions. The phase noise values of the generated RF signals are kept to be around −105 dBc/Hz@10 kHz. [ABSTRACT FROM AUTHOR]

One of the serious limitations in frequency-modulated continuous wave light detection and ranging (FMCW-LiDAR) is the maximum ranging distance as imposed by the coherence of light sources. The author proposes and demonstrates a new technique for overcoming the limitation using differential detection scheme between the frequency-fixed carrier and frequency-modulated subcarrier, which are generated from a single laser source. In the proof-of-principle experiment, two reflection points with a 3 m separation are clearly resolved at distances 192.2 m and 1.504 km using a distributed feedback (DFB) laser with the coherence length of 70 m. [ABSTRACT FROM AUTHOR]

The performance of a frequency‐modulated continuous‐wave light detection and ranging is severely limited by the inherent non‐linear chirp when laser diodes with direct frequency modulation are used as light sources. The author proposes and demonstrates a new modulation and detection scheme that is immune to non‐linear chirp. Sinusoidal frequency modulation and beat phase detection are employed for measuring both the velocity and distance of a target. Relative accuracies below ± 2 mm/s and ± 1 mm are confirmed for velocity and distance measurements, respectively, in the proof‐of‐principle experiment. [ABSTRACT FROM AUTHOR]

The performance of a frequency‐modulated continuous wave (FMCW)–light detection and ranging (LiDAR) is severely limited by the inherent non‐linear chirp when laser diodes with direct frequency modulation are used as light sources. To overcome this limitation, optoelectronic feedback techniques were reported for chirp linearisation. The author propose and demonstrate an alternative technique that is based on regression analysis of FMCW–LiDAR beat signals. It is shown that the waveform of the laser frequency modulation accompanied by the non‐linear chirp is analytically expressed in terms of the Fourier series containing both the odd and even harmonics. The analytical expression is utilised in the regression analysis to calculate the unknown distances. A relative accuracy below 1 mm is obtained in the proof‐of‐principle experiment. [ABSTRACT FROM AUTHOR]

The authors present an expanded porous polytetrafluoroethylene terahertz fibre transmission link using resonant tunnelling diodes integrated with photonic‐crystal waveguides as a transmitter and a receiver. The measured coupling efficiency between the terahertz fibre and the photonic‐crystal waveguide integrated with a silicon tapered coupler is ∼30% with a 3 dB bandwidth of 50 GHz at the 0.3 THz band. Error‐free 10 Gbit/s transmission and uncompressed 4 K high‐definition video transmission have been successfully demonstrated using 1 m‐long terahertz fibre. [ABSTRACT FROM AUTHOR]

Transmission characteristics of an optical signal, which is converted from a high‐speed terahertz‐wave signal with high spectral efficiency, are investigated and reported. A Nyquist time division multiplexing 40 Gbit/s on–off keying signal in the terahertz band is generated with high‐speed photo‐mixing. The generated terahertz‐wave signal is again converted into an optical signal with heterodyne detection, optical intensity modulation, and optical filtering. Then the chromatic dispersion tolerance of the converted optical signal is investigated in order to obtain knowledge for realising the connection between the fibre‐optic and the wireless links. After briefly explaining the conversion method, some related experimental results are reported. The results include evaluation of chromatic dispersion tolerance with regard to the optical signal at the final stage. As a sequel to bit error rate measurement, the optical signal can be transmitted to a 9 km‐long single‐mode fibre without chromatic dispersion compensation. [ABSTRACT FROM AUTHOR]

The authors propose and experimentally demonstrate an optical vector network analyser (OVNA) employing optical double‐sideband modulation and optical Hilbert transform. The optical device‐under‐test is characterised with and without optical Hilbert transform. Then, by employing post signal processing, accurate spectral responses on upper and lower sides of the optical carrier are observed. A proof‐of‐concept experiment for measuring gain and absorption responses of the stimulated Brillouin scattering (SBS) is performed, showing that the proposed OVNA is able to accurately measure the gain and absorption responses of the SBS. The measurement error is smaller than 1%. [ABSTRACT FROM AUTHOR]

4H‐SiC ultraviolet p‐i‐n photodiodes with four different epitaxial structures were fabricated. The experimental results prove that both a thin P+‐type Ohmic contact layer and a thick intrinsic layer were indispensible for a high‐performance ultraviolet p‐i‐n photodiode. A 4H‐SiC p‐i‐n photodiode with responsivity as high as 0.139 A/W at 278 nm incident wavelength was achieved. Meanwhile, within a certain wavelength range, the peak response wavelength of an ultraviolet p‐i‐n photodiode would be modulated by properly varying the thicknesses of P+‐type layer and the intrinsic layer. Moreover, the theoretical calculation was carried out to further authorise the experimental results. [ABSTRACT FROM AUTHOR]

Electrode‐less integrated optics lithium niobate (LiNbO3) asymmetric Mach–Zehnder interferometers (AMZIs) can be used as electric field (e‐field) sensors. Electrode‐less sensors are attractive for practical use as such devices do not perturb the sensed variable. A critical issue when using electrode‐less sensors is to ensure a linear sensing process. This requires the electro‐optic transfer function (EOTF) to be adjusted at its quadrature point. For this purpose, an electrode‐less sensor can be optically biased. However, an accurate optical bias is strongly limited by the fabrication tolerances of the AMZIs. Hence a linear sensing‐detection process is not actually ensured. To explore a linear e‐field sensing‐detection process, a simple linearising scheme is proposed in this Letter. The scheme is configured by cascading two optically matched AMZIs. The first AMZI is an electrode‐less sensor; the second one, which is provided of electrodes performs simultaneously as an optical demodulator and an EOTF tuner. The EOTF of the demodulating AMZI can be tuned to its quadrature point by a DC voltage on its associated electrodes. The sensing‐detection scheme is modelled and experimentally tested when illuminated either by coherent and incoherent light. The sensed e‐field is linearly recuperated only when incoherent light is used. [ABSTRACT FROM AUTHOR]

A scheme for simultaneous down‐conversion and linearisation of radio‐frequency (RF) signals is proposed and demonstrated. Based on carrier‐suppressed double‐sideband modulation, the input RF signal can be down‐converted to any intermediate frequency (IF) tone in the range of 8–12 GHz. Compared with the down‐conversion scheme with two cascaded intensity modulators, the proposed scheme has improvement of >21 dB of the suppression ratio of third‐order intermodulation distortion and 6.17 dB of spurious‐free dynamic range. [ABSTRACT FROM AUTHOR]

A novel scheme for the generation of an optical frequency comb with exceptionally high flatness for a large number of spectral lines is proposed. The scheme consists of a dual‐parallel Mach–Zehnder modulator (DP‐MZM) with which to carve the narrow intensity pulse and phase modulators for the application of high chirp. The experimental results show perfect spectral profiles with 35 lines with <1 dB flatness and 54 lines at 1.6 dB flatness when a DP‐MZM is combined with two and three phase modulators, respectively. [ABSTRACT FROM AUTHOR]

The authors used 65‐nm CMOS technology to develop a linear four‐channel driver IC with low temperature dependence and ultra‐low power dissipation for 64‐Gbaud coherent optical transmitters. The driver showed more than a 48‐GHz 3‐dB electrical bandwidth and less than 1‐W power consumption in four‐channel operation. By employing a circuit that suppresses the temperature dependence, they achieved 3‐dB electrical bandwidth variation of 3.0 GHz and the gain variation of 1.5 dB under the −5 to 75°C and ±5% supply voltage variation conditions. The CMOS driver has all the necessary functions for a high‐bandwidth coherent driver modulator such as a gain control, peaking control, peak detection and temperature monitoring, all of which functions can be controlled by a serial peripheral interface. A fabricated sub‐assembly consisting of the CMOS driver and an InP modulator showed a 48‐GHz 3‐dB electro‐optic bandwidth. [ABSTRACT FROM AUTHOR]

Successful experimental demonstration and field trial of 400GBASE‐LR8 is reported based on 4‐level pulse amplitude modulation format over standard G.652 single‐mode fibre using CFP8 pluggable optical transceivers. Stable long‐term error‐free performance of 400GBASE‐LR8 optical signal over fibre spools of varying lengths up to 35 km is demonstrated, which is significantly longer than the IEEE standard 802.3bs specification of 10 km reach. For the first time, error‐free full throughput performance of 400GBASE‐LR8 over a 27 km field fibre link including optical patch panels through operational exchanges is also demonstrated. The results demonstrate that this early 400GBASE‐LR8 client technology is suitable for practical network deployment scenarios and has potential for even longer reach beyond the 10 km specification. [ABSTRACT FROM AUTHOR]

The authors propose a transmitter digital filter for joint optimisation of signal pulse shaping and chromatic dispersion pre‐compensation, providing a detailed analysis of the architecture complexity. Experimental transmission of 40 × 50G channels (28 Gbaud four‐level pulse amplitude modulation) over up to 110 km uncompensated standard single mode fibre with low‐bandwidth hardware is demonstrated. [ABSTRACT FROM AUTHOR]

The free‐space optical communication of high‐speed 120 Gbit/s wavelength‐division multiplexing (WDM) QPSK signals is demonstrated in an outdoor atmospheric channel with a distance of about 1 km. A pair of optical terminal systems with the capture and tracking functions is used to establish the atmospheric channel. The power fluctuation through the atmospheric turbulence agrees well with the exponentiated Weibull distribution. At the BER of 3.8 × 10−3, the required receiving powers for the three WDM channels are similar, which are all <−15.4 dBm. [ABSTRACT FROM AUTHOR]

The continual increase in demand for extremely high data‐rate systems spans the full range of network systems from local area networks to long‐haul systems. While coherent optical systems dominate the long‐haul market, there is significant interest in using simpler direct detection systems for application areas, where simplicity and lower‐cost is a key driver. Pulse‐amplitude modulation (PAM) is appropriate to consider as it enables the reduction of the channel symbol rate for a given data rate, without unduly increasing the complexity of the system. A route to the design of optimum post‐detection filter for PAM systems is presented that offers significantly improved robustness to timing jitter, which is an impairment of increasing import as symbol rates are increased. [ABSTRACT FROM AUTHOR]

The authors propose and demonstrate a photonics‐based wideband Doppler frequency shift (DFS) measurement scheme through in‐phase and quadrature detection. The system has a compact structure applying cascaded phase modulation and polarisation modulation. It is easy to implement since both the value and sign of the DFS can be estimated by simple signal processing. More importantly, the proposed scheme is suitable for both DFS measurement and direction discrimination over a very wide frequency range. In the experiment, accurate DFS measurement with a large carrier frequency range from 5 to 40 GHz is demonstrated with the measurement error kept within ±12 Hz. [ABSTRACT FROM AUTHOR]

The authors compare transmission reach of 100 Gb/s four‐level pulse amplitude modulated signals over single‐span systems constructed with three optical fibre types. They demonstrate reach up to 160 km with fibres having ultra‐low attenuation, one with a large effective area. [ABSTRACT FROM AUTHOR]

A technique is experimentally demonstrated to double the frequency of a microwave signal by synchronising and polarisation multiplexing the optical signals from a dual‐output Mach–Zehnder modulator. The frequency‐doubled tone at 4 GHz is 41.2 dB stronger than the residual fundamental‐frequency tone. The strongest side tone is found at 12 GHz (sixth order) and is suppressed by 32.7 dB. The phase noise of the frequency‐doubled signal is −104.1 dBc/Hz at an offset of 10 kHz, an increase of 6.7 dB over that of the initial signal. This technique does not rely on electronic or optical filters or amplifiers and is demonstrated using commercial off‐the‐shelf components. [ABSTRACT FROM AUTHOR]

Operation of single‐stage, single‐pump E‐band bismuth‐doped fibre amplifier with gain, noise figure and efficiency comparable to the state‐of‐the‐art erbium‐doped fibre amplifier of the same complexity and the gain bandwidth of 50 nm is demonstrated. The amplification of 10 Gbit/s intensity modulation/direct detection signal with power penalty below 1.5 dB is reported. [ABSTRACT FROM AUTHOR]

Recently, the authors proposed a physical‐layer key generation scheme for optical orthogonal frequency division multiplexing (OFDM) techniques in an indoor scenario. The key generation protocol exploits bipolar and real‐valued optical OFDM samples to create confidential keys. It determines a chunk of cyclic prefix samples positioned in the space of small channel impact to generate the confidential keys during the session duration. This key‐generation approach offers strong computational security to counter passive eavesdroppers and key creation on the fly, with zero bit‐mismatch rates. In this Letter, the authors evaluate the randomness of the confidential keys produced from the recently proposed key‐generation protocol to validate if the keys are truly random. The authors apply a statistical test suite of the National Institute of Standards and Technology to test randomness. The results demonstrate that the keys generated from the proposed key generation system pass all the tests. The results also demonstrate that the proposed key generation system provides uncorrelated keys with an approximately equal probability of bits (discrete uniform distribution) on an average, which yields a high security system against statistical attacks. [ABSTRACT FROM AUTHOR]

A real‐time processing method for demultiplexing a high‐speed terahertz (THz)‐wave OFDM signal is proposed and demonstrated. An electrical signal, which is down‐converted from the THz‐wave OFDM signal, is up‐converted into an optical OFDM signal through an optical modulator. Then, the generated optical OFDM signal is demultiplexed into sub‐carrier channels in the optical domain by utilising an integrated‐optic OFDM demultiplexer with discrete Fourier transform function. The processing principle of the method and some preliminary experimental results including BER measurement to demonstrate the effectiveness of the method are reported. A 2 × 10 Gbit/s THz‐wave OFDM signal was demultiplexed into sub‐carrier channels with the method as preliminary investigation. [ABSTRACT FROM AUTHOR]

A photonic technique for microwave frequency measurement based on stimulated Brillouin scattering (SBS) with low measurement error and large measurement range is demonstrated. In the scheme, an optical phase modulator is employed and the intensity balance between the two sidebands of the phase‐modulated signal is broken by use of SBS in a fibre. The frequency measurement is achieved by detecting the existence of a microwave signal after photoelectric detection of the phase modulated signal. The measurement error is experimentally shown to be 50 MHz in a narrow measuring range of ∼1 GHz and 100 MHz in a wide range of 20 GHz. [ABSTRACT FROM AUTHOR]

A 100‐Gbit/s PAM‐4 (pulse amplitude modulation) transmission over 4‐km standard single‐mode fibre (SSMF) and a C‐band 4 × 80‐Gbit/s PAM‐4 transmission over 8‐km standard SSMF experimentally demonstrated. A digital Wiener filter (WF) at the receiver side is utilised to compensate the impairments introduced by the transmission link. Bit error rate below 7% forward error correction threshold (3.8 × 10−3) is achieved with received optical power of 0 dBm for the single lane 100‐Gbit/s transmission and around −4 dBm for the four channels after 8‐km transmission with WF, which is over one order of magnitude better than that without equalisation. The results indicate that WF is a good enough option for practical application in short reach optical interconnect. [ABSTRACT FROM AUTHOR]

Absolute added correlative coding with eight levels has been proposed and demonstrated numerically at bit rates of 30 and 60 Gbps back‐to‐back, and up to 75 Gbps over 2000 and 3000 m standard single‐mode fibre using a single‐drive multiple quantum‐well Mach–Zehnder modulator. The transmission performance is compared with the conventional eight‐level pulse amplitude modulation. [ABSTRACT FROM AUTHOR]

The use of a 35 GHz reflective electroabsorption modulator (REAM) monolithically integrated with a semiconductor optical amplifier (SOA) for indoor wideband analogue applications is demonstrated. Unlike REAMs, good performances over a wide range of input optical power and wavelength are observed, which mitigates the need for accurate control of the input signal, especially at remote sites where simplicity is strictly required. In addition, it provides amplification functions to compensate for the modulator insertion losses. The device exhibits an RF gain in excess of −22 dB and spurious‐free dynamic range of >103 dB Hz4/5. Implementing the rate‐adaptive discrete‐multitone modulation, 35 Gbit/s aggregate transmission speed is demonstrated for a short‐range wideband subcarrier multiplexed radio‐over‐fibre system. [ABSTRACT FROM AUTHOR]

This Letter presents a novel CMOS transceiver for short‐reach multigigabit communication over step‐index plastic optical fibre (SI‐POF), a very interesting transmission medium for industrial and home‐end applications due to its low cost, ease of handling and inherent immunity to electromagnetic interference. The transceiver uses amplitude duobinary modulation, which is added to continuous‐time equalisation. For that purpose, a duobinary precoder and a duobinary decoder have been fabricated in a standard 0.18‐µm CMOS technology, consuming only 28.4 mW. The system enables a 50‐m SI‐POF channel bandlimited to 100 MHz to operate at a data bit rate of 3.125 Gbit/s. [ABSTRACT FROM AUTHOR]

The performance of a typical free‐space optical link using a recently proposed composite model for turbulence and pointing error effects is studied. This model offers analytical tractability in the statistical description of the above phenomena and was derived using the mixture Gamma distribution for turbulence. New analytical derivations of the outage probability and bit‐error rate for some binary modulation schemes are presented in closed form. [ABSTRACT FROM AUTHOR]

Spectral profile control method of a dispersion‐tuned fibre laser is demonstrated. A dispersion‐tuned fibre laser is a fast and widely tunable wavelength‐swept laser without mechanical tunable filters. The lasing centre wavelength is tuned by changing the active mode‐locking modulation frequency to that of the corresponding operating wavelength matching the fundamental repetition rate of the cavity. The sweep spectra profile can be controlled by modulation frequency sweep. By applying a pre‐calculated modulation frequency sweep, the flatness of the wavelength‐swept spectral profile can be improved. We applied the dispersion‐tuned wavelength‐swept laser to a spectral‐domain optical coherence tomography system as a wideband light source and successfully obtained OCT images. The image resolution is enhanced by applying a nonlinear modulation frequency sweep to the laser instead of linear modulation frequency sweep. The potential of the proposed dispersion‐tuned wavelength‐swept laser as a spectrally flexible laser source is demonstrated. [ABSTRACT FROM AUTHOR]

A fast detection system based on a terahertz quantum‐well photodetector (QWP) and a fast modulated terahertz quantum‐cascade laser is presented. Response waveforms and eye diagrams under different modulation frequencies are acquired. Employing the current measurement system without any high speed optimisation obtains the highest response rate of 0.5 GHz. The experiment demonstrates that the QWP is potentially useful for fast detection and high speed communication applications. [ABSTRACT FROM AUTHOR]

An optical two‐tone signal with 60 GHz frequency spacing is experimentally demonstrated, by sinusoidally modulating two lightwaves with different modulation indices and RF phases. By superposing two modulation lightwaves with orthogonal polarisation, polarisation orthogonality is achieved between undesired low‐order optical sidebands and the desired high‐order optical sidebands together with the incident light‐wave. This enables us to suppress the undesired sidebands using polarisers instead of optical filter. Such operation has been implemented using optical modulator possessing RF ports for external termination, which is nested in one mode of polarisation‐maintaining Sagnac interferometer (PMSI). This implementation is equivalent to optical single‐sideband modulator, but required nested Mach–Zehnder structure is just one. Owing to this reduction together with use of one mode in PMSI, long‐term stable operation more than 6 h has been successfully obtained, with 29 dB suppression ratio of undesired first‐order sidebands against desired third‐order optical sidebands. [ABSTRACT FROM AUTHOR]

The transmission of multiple standard‐compliant long‐term evolution advance (LTE‐A) bands together with a 4‐pulse amplitude modulation (PAM) baseband signal over 50‐m‐long 1 mm core diameter polymethyl methacrylate graded‐index plastic optical fibre is demonstrated. Transmission of eight LTE‐A 64‐QAM bands and a 1.2 Gbit/s 4‐PAM baseband signal over the fibre was achieved resulting in an error vector magnitude <8% and pre‐forward error correction (FEC) BER < 10−3, respectively. [ABSTRACT FROM AUTHOR]

A 31.2 dB‐contrast optical AND gate based on a Fabry–Pérot semiconductor optical amplifier is experimentally demonstrated, exceeding previously demonstrated data for such a device by over 24 dB. Whereas cross‐phase modulation shifts the Fabry–Pérot resonance to trigger the AND‐gate functionality, cross‐polarisation modulation enables ultrahigh contrast by producing a state of polarisation (SOP) for the high output‐power state that is different than that of the low output‐power state; a linear polariser is then used to block the low‐power SOP. This technique is applicable to other resonator geometries, Kerr‐nonlinear media, and to other all‐optical combinational‐logic gates. [ABSTRACT FROM AUTHOR]

Successful 54 Gbit/s on–off keying (OOK) transmission over 2.2 km of multimode OM4 fibre is demonstrated. The decisive advantages of the single mode over multimode vertical cavity surface emitting laser (VCSEL) in transmission experiments are shown. [ABSTRACT FROM AUTHOR]

A hardware‐efficient adaptive algorithm for frequency offset and phase noise mitigation in coherent optical quadrature amplitude modulation systems is presented and analysed. Hardware efficiency for the mitigation of imperfections is achieved by computing directly the complex exponential, thus avoiding a CORDIC processor in the phase recovery loop. The design includes a square‐root‐ and trigonometry‐free update. Simulation results substantiate the theoretical findings. [ABSTRACT FROM AUTHOR]

Colour shift keying (CSK) can achieve significant performance gains in multi‐colour visible light communications (VLCs). Existing CSK constellations are designed within the positive domain which satisfies the requirement of non‐negative optical intensity modulations. A scheme is proposed to incorporate spatial modulation with a CSK design by grouping LEDs. In this way, the CSK constellation is optimised within the entire real domain and hence achieves a larger minimum Euclidean distance so as to enhance the bit error rate (BER) performance. Simulation results show a noticeable performance gain by the proposed scheme compared with traditional CSK under ideal light‐of‐sight optical channels. Further, by experimental tests, an even more pronounced BER gain is achieved by the proposed scheme over real VLC channels. [ABSTRACT FROM AUTHOR]

A wideband tunable single bandpass microwave photonic filter (MPF) based on the four‐wave mixing (FWM) nonlinear dynamics of a distributed feedback (DFB) semiconductor laser under optical injection is proposed and experimentally demonstrated. Utilising the wavelength‐selective optical gain of the optical‐injected DFB laser, phase‐modulation to intensity‐modulation conversion is obtained, leading to the realisation of a single bandpass MPF. By changing the injection parameters, an MPF with a frequency tuning range of 27.72 GHz has been achieved. The minimum 3 dB bandwidth and the out‐of‐band suppression ratio are measured to be 61.2 MHz and 25 dB, respectively. [ABSTRACT FROM AUTHOR]

The article discusses how probabilistic shaping (PS) is a powerful technique for optical communications, providing both continuous data rate adaption and shaping gain. It mentions that PS signals can reach higher overall data capacity and longer transmission distance compared to the conventional QAM signals.

A novel THz‐wave autocorrelator consisting of an optical delay line and high‐speed photodiodes was proposed for the measurement of optical waveforms with a high rate of repetition. In this system, the waveform is delayed at the optical domain and correlated at the THz domain. Experimental results show clear autocorrelations of 100, 200 and 300 GHz sinusoidal waveforms which indicate the feasibility of the proposed method. [ABSTRACT FROM AUTHOR]

A technique to estimate an incoming radio frequency (RF) signal angle of arrival (AOA) irrespective of the RF signal amplitude is presented. It is based on measuring the DC voltage at the output of two modules where each of them consists of an optical modulator, an optical filter and a photodetector. An RF signal amplitude‐independent AOA can be obtained from the ratio of the two DC voltages. Experimental results are presented that demonstrate 0°–63° AOA measurement with <3° errors for an RF signal with three different power levels of 3.2, −0.3 and −3.6 dBm into the modulator. [ABSTRACT FROM AUTHOR]

Orthogonal frequency division multiplexing (OFDM) systems have inferior nonlinear performance due to their high peak to average power ratio (PAPR) characteristics, which degrades the performance in optical wireless communication systems. A modified asymmetrically clipped optical (ACO)‐OFDM system with low PAPR by introducing a discrete sliding norm transform (DSNT) as low‐complexity post‐inverse discrete Fourier transform processing is proposed. Numerical results show that the proposed DSNT‐ACO‐OFDM achieves about 50% PAPR reduction, while maintaining a competitive bit error rate performance compared with the conventional schemes. [ABSTRACT FROM AUTHOR]

The performance assessment of a novel multilevel modulation format based on partial‐response signalling called absolute added correlative coding (AACC) by numerical simulations is delineated, targeting short‐range and interconnect high‐speed optical networks. The spectral efficiency, chromatic dispersion tolerance and receiver sensitivity of 4‐instensity level AACC are discussed and compared against other well‐known existing modulation formats, namely, non‐return‐to‐zero on–off keying and 4‐ary pulse‐amplitude modulation (4‐PAM). The robustness of the higher order of AACC over 8‐PAM and 16‐PAM is also discussed. [ABSTRACT FROM AUTHOR]

Temperature‐stable oxide‐confined 980 nm vertical‐cavity surface‐emitting lasers for optical interconnects are presented. For the first time 40 Gbit/s 4‐PAM data transmission is experimentally demonstrated at temperatures of 100 and 120°C. [ABSTRACT FROM AUTHOR]

A novel chaos‐based selected mapping (SLM) scheme is proposed for physical layer security in OFDM‐PONs, where a Hénon chaos map is employed to create a large key space for OFDM data encryption. The transmission performance is significantly improved due to the peak‐to‐average power ratio reduction in the SLM scheme. High quality 8.9 Gbit/s 16‐QAM OFDM encrypted data transmission is demonstrated over a 20 km single‐mode fibre. [ABSTRACT FROM AUTHOR]

A high‐speed and energy‐efficient oxide‐confined 850 nm vertical‐cavity surface‐emitting laser (VCSEL) for optical interconnects is presented. A record‐high modulation bandwidth of 30 GHz is reached for a 3.5 µm oxide aperture VCSEL, with 25 GHz bandwidth already at a bias current of 1.8 mA. The high bandwidth at low currents enables energy‐efficient transmission with a dissipated heat energy in the VCSEL of <100 fJ/bit at 25, 40 and 50 Gbit/s. [ABSTRACT FROM AUTHOR]