Iet Optoelectronics最新文献

For a serial relaying underwater wireless optical communication (UWOC) system with ON-OFF keying modulation, we theoretically evaluate the optimal power allocation techniques in order to minimise the end-to-end bit error rate (BER), subject to transmission power constraints. At first, we evaluate the end-to-end BER with respect to all degrading effects of the UWOC channel, namely scattering, absorption, and turbulence-induced fading and then develop a closed-form BER expression as a function of transceiver parameters and water type. The optimal power allocation methods are obtained using the perfect channel state information available at the receiver (CSIR) and transmitter (CSIT) for both detect-and-forward (DF) and amplify-and-forward (AF) serial relaying systems. For each relaying method, we consider a dual-hop UWOC system and determine optimal power allocation to minimise the BER. Afterwards, the optimal power allocation in a multi-hop system is obtained to minimise the end-to-end BER. Compared to the equal power allocation, our results illustrate that UWOC relaying systems with optimal power allocation can significantly improve the end-to-end BER and expand the communication link. For instance, the proposed power allocation method for the DF and AF relay node in a 60 m single relay system improves the system performance at the BER of 10⁻⁵ by 2.5 and 1.8 dB compared to the equal power allocation, respectively.

SF₆ has been recognized worldwide as the main insulating gas for Gas-Insulated Switchgear (GIS). It is often required to accurately and effectively detect typical SF₆ decomposition. In this paper, a sub-ppm-level SO₂F₂ and SOF₂ gas sensor based on photoacoustic spectroscopy (PAS) is proposed and demonstrated. The steel resonant photoacoustic cell with a resonant frequency of 1750 Hz was designed and fabricated. The harmonic detection technique in wavelength modulation spectroscopy was applied to improve the signal-to-noise ratio. A mercury-cadmium-telluride photodetector was added in the PAS system to monitor the input optical power, in order to compensate for the errors caused by power fluctuations. SO₂F₂ and SOF₂ were measured by using distributed feedback quantum cascade lasers at 6648 and 7463 nm, respectively. The results show that the proposed sensor performed with favourable linearity within the dynamic range, and reached the minimum detection limit of 0.22 ppm for SO₂F₂ and 0.28 ppm for SOF₂, which indicated its great potential for monitoring the decomposition process of SF₆ in GIS.

A 1 × 48 large-scale multi-mode fibre (MMF) optical switch driven by a stepper motor is reported. The insertion loss is <1.42 dB, which can be further reduced to 1.02 dB after optimisation. With the employment of a corner cube prism in the optics, the device is characterised by good repeatability of <±0.01 dB. Meanwhile, the device has a fast response of <8 ms.

This study proposes a long-period fibre grating (LPFG) curvature estimation method based on random forest regression (RFR) to address the shortcomings of the existing curvature evaluation method, namely, polynomial fitting; these shortcomings cause difficulty in achieving adequate model regularity and application universality. The resonant wavelength and resonant peak amplitude of the LPFG are used as input variables in this method to develop an RFR model for curvature estimation, allowing for accurate curvature prediction of the sample. The results show that the RFR-based LPFG curvature prediction model can better characterise the input–output regression relationship than back-propagation neural networks. The average R² value of the RFR model is 0.9826, and the actual measured curvature value is highly correlated with the model predicted curvature value. Compared to that exhibited by back-propagation neural networks, the RFR model exhibits higher accuracy for curvature estimation, with average values of 0.1314 and 0.1173 for root mean square and mean absolute errors, respectively. This method can provide a more comprehensive theoretical basis for the application of robot learning in the curvature measurement of LPFG and has practical value.

Visible light communication (VLC) has emerged as a good accompaniment to radio-frequency (RF) technologies by deploying multicarrier schemes such as orthogonal frequency division multiplexing (OFDM). However, the coherent summation of carriers in the OFDM system leads to a high peak-to-average-power ratio (PAPR), causing non-linear clipping distortion at the transmitting light-emitting diode. This intricacy becomes a potential barrier for intensity modulation and limits the VLC systems' bandwidth. In the literature, non-distorting PAPR lowering approaches, such as the selective mapping (SLM) approach, have been confirmed as the most effective strategy for reducing ineludible high PAPR in optical OFDM systems among all other available techniques. Besides its astounding performance, the computational complexity also becomes a major complication in SLM due to the generation of multiple candidates. This paper proposes a computational complexity minimisation approach using inherit system properties in the SLM-based PAPR suppression method for VLC systems, where the phase sequence vectors are considered periodic. The alternative direct current-biased optical orthogonal frequency division multiplexing candidates for the single frequency-domain data block are generated with the periodic phase rotation vector. The evaluation of the complexity analysis and the simulation results shows that the mitigation of computational complexity surpasses the standard SLM technique.

A scheme for multi-band frequency generation is proposed and experimentally demonstrated using a dual-parallel Mach–Zehnder modulator (DPMZM)-based optoelectronic oscillator (OEO). The principle of this generator is theoretically described in detail. It is found that multiple frequencies with a small power variation can be produced by setting the three working points and the modulation depth of the DPMZM. In the proving experiment, an 8-GHz fundamental oscillation with a side mode suppression ratio of 44 dB is generated. The phase noise is measured to be −96.8 dBc/Hz @ 10 KHz offset with a 100-m and 1-Km optical fibre used in the two oscillation loops. When the oscillating signal is applied to drive the two sub-modulators in the DPMZM, doubled and tripled frequencies are produced at the same time, and the phase noises of the two frequencies are also measured to be −90.2 dBc/Hz and −85.9 dBc/Hz @10 KHz, respectively.

All-optical likelihood calculation has the potential to improve the performance of operating speed and power consumption in future communication systems. The author previously proposed a novel scheme of likelihood calculation, which is capable of applying multi-value modulation. However, these studies have a limitation that requires higher integration into an actual system. In this study, an optical device of likelihood calculation for a 4-bit quadrature phase-shift keying (QPSK) modulated signal (i.e. two sequential QPSK symbols) is demonstrated. The integrated device consists of two delayed interferometers with silicon waveguide. The device is designed according to the types of 4-bit code string as follows (00 00), (00 11), (11 00), and (11 11), and the output light waveform from the device is observed by an oscilloscope for a plurality of 4-bit QPSK signals. The light intensity obtained from the device accurately corresponds to the Hamming distance between the code string and the input signal. The results indicate that the proposed scheme correctly calculates a likelihood for a 4-bit QPSK signal at 10 Gbaud.

The microring modulator (MRM) is a small-size and low-power component, which is the potential for the next-generation optical interconnection. By the theoretical analysis, increasing the electrical bandwidth, which is relevant to the cross section and the doping concentration of the doping region, is a better way to increase electro-optical (EO) bandwidth. Therefore, a new doping profile of the PN depletion region with 4 doping concentration levels is introduced considering the electrical bandwidth. Based on the new doping profile, the MRM with the 160 μm length and the 0.33 and 0.2 μm coupling space are determined considering the trade-off between the EO bandwidth, quality factor (Q), extinction ratio, area of MRM, and power consumption. Moreover, the MRM is characterised and is applied in a wavelength division multiplexing transmitter. By the measurement, the transmitter with the designed MRM could transmit PAM4 signal at 52 Gbps rate, which indicates the MRM could be potential for the 50 Gbps/ch optical interconnection.

In this paper, we present a GST-based wideband plasmonic switch. With the excitation of localised surface plasmons and concentration of electric field in the structure, a near-perfect and wideband absorption is achieved. The switch consists of a GST layer, which acts as a Fabry–Perot cavity. Increasing the temperature and changing the state of GST lead to a high difference between the absorption spectra in amorphous and crystalline states in a wide range of wavelength. Therefore, the switch has a high extinction ratio of 13.71 dB at the wavelength of 1343 nm. Also, the response time of the switch is obtained as 46 fs. The structure has near-perfect absorption up to the incident angle of about 20°. Moreover, due to the symmetry of structure, the absorption spectrum is independent of polarisation. To show the validity of simulation results, the analytical method of equivalent circuit model is presented. The proposed polarisation-insensitive switch with high extinction ratio, fast response time and wide bandwidth can be used in photodetectors, plasmonic modulators and logic gates.