High-spectral-efficiency optical direct detection using the stokes vector receiver

We introduce a three-dimensional modulation format using a Stokes-vector direct-detection system, where a pulse amplitude modulation (PAM) signal is polarization multiplexed with a quadrature amplitude modulation (QAM) signal. The phase reference of the PAM signal enables the retrieval of the phase information of the QAM signal using a Stokes vector receiver (SVR). A dual-polarization in-phase–quadrature modulator is employed to generate QAM-PAM optical signals with chromatic dispersion precompensation. We show direct detection of dual-polarization 16QAM-PAM2 and 16QAM-PAM4 signals delivering 280 and 336 Gb/s (net bit rate 233 and 280 Gb/s) below the soft-decision forward error correction threshold of 2 $\times$ 10 $^{-2}$ at 320 and 80 km, respectively. In addition, we study system performance at different modulation formats and symbol rates. Our findings indicate that increasing the order of modulation formats of the QAM signal is better than increasing that of the PAM signal to achieve the same bit rate. Finally, the performance of a 56 Gbaud 16QAM signal with PAM2 modulation on one polarization and a 56 Gbaud 16QAM signal with a pilot carrier on the orthogonal polarization detected with an SVR is experimentally investigated.

Stokes vectors direct detection (SV-DD) is an effective solution for short-reach optical communications.In this paper, we investigate 2D-modulation direct detection systems based on three Stokes vector receivers (SVRs).The influences of three key factors, including the states-of-polarization (SOP), the splitting ratio of the coupler, and the excess loss (EL), are studied in detail.It is shown that the splitting ratio for achieving optimum performance will be changed with SOP and EL conditions.Among these SVRs, the 3 × 3 coupler-based receiver with its optimal splitting ratio shows the best BER performance and stability against the change of SOP.

We propose a hybrid optical Hilbert receiver and Stokes vector receiver (OH-SVR) capable of phase and polarization diversity. We experimentally demonstrate single-wavelength direct detection at 1.6-Tb/s and 1.56-Tb/s line rates at BTB and after 10-km standard single-mode fiber transmission for data-center interconnects.

According to different transmission distances, application scenarios of a data center mainly include intra- and inter-data center optical interconnects. The intra-data center optical interconnect is considered as a few kilometers optical interconnect between servers and racks inside a data center, which accounts for nearly 80% of data traffic of a data center. The other one, inter-data center optical interconnect, is mainly applied in tens of kilometers data transmission among different data centers. Since data exchange in data centers generally occurs between many servers and racks, and a lot of transmitter and receiver components are required, optical interconnects become highly sensitive to component costs. In this paper, we firstly review the development and applications of mainstream transmitter components (e.g., VCSEL, DML, EML, MZM, and monolithic integrated transmitter) and receiver components (e.g., single-end photodetector, Kramers-Kronig receiver, Stokes vector receiver, and monolithic integrated receiver), which have been widely applied in short-reach transmission systems. Then, two types of integrated solutions including simplified detection scheme and transceiver integration scheme are presented in detail. Finally, we summarize and discuss the technological and component options for different transmission distances. We believe that monolithic integrated components, especially transceiver integration, will become a powerful solution for next-generation high-speed short-reach transmission systems.

Signal and Quantum Noise in Optical Communications and Cryptography

A novel phased array receive antenna steering system is introduced. The core of this system is an optical ring resonator-hased broadband, continuously tunable optical beam forming network (OBFN). In the proposed system architecture, filter-based optical single-sideband suppressed-carrier modulation and balanced coherent optical detection are used. Such architecture has significant advantages over a straightforward architecture using optical double-sideband modulation and direct optical detection, namely relaxed bandwidth requirements on the optical modulators and detectors, reduced complexity of the OBFN chip, and enhanced dynamic range. Initial measurements on an actual 1 times 8 OBFN chip and an optical sideband filter chip arc presented. Both are realized in CMOS-compatible planar optical waveguide technology.

The detection technology of infrared polarization has gained significant attention due to its ability to provide better identification and obtain more information about the target. In this paper, based on the expression of the full polarization state in Stokes space, we designed micro-nano metasurface functional arrays to calculate the polarization state of the incident light by reading the Stokes parameters (a set of parameters that describe the polarization state). Metalens with linear and circular polarization-dependent functions are designed based on the propagation and geometric phases of the dielectric Si meta-atoms in the infrared band, respectively. The device exhibits a high polarization extinction ratio. The influence of incident angle on polarization-dependent metalens is discussed, and the analysis of incident angle is of great significance for the practical application. An infrared six-foci metalens is proposed, each corresponding to the Poincaré sphere's coordinate component (a graphical polarization state method). By matching the six polarization components of the incident light and the Stokes parameters, the polarization detection function can be realized by calculating the polarization state of the incident light. There is a slight error between the theoretical value and the calculated value of the unit coordinate component of the Stokes parameters. At the same time, the intensity distribution of different incident light polarization azimuth angles and ellipticity angles on the focal plane agrees with the theory. The advantage of the device is that the polarization state of the incident light can be directly calculated without passing through other components. The six-foci metalens have potential applications in polarization detection and imaging, space remote sensing, etc.

Spectral efficient frequency division multiplexing (SEFDM) can offer a higher spectral efficiency (SE) than orthogonal frequency division multiplexing (OFDM). In this work, we propose a diversity technique based on SEFDM for beyond 100-Gb/s optical intensity modulation and direct detection (IM/DD) long reach (LR) applications. We mathematically demonstrate that the self-created inter-carrier interference of SEFDM signals can be reused to achieve a diversity gain on each sub-carrier and, in turn, improve the tolerance to power fading induced by chromatic dispersion (CD) in IM/DD LR links. Based on the proposed diversity technique, we further demonstrated a 112-Gb/s SEFDM transmission over 80-km standard single-mode fiber, using only 28-GHz bandwidth and modulation format of up to 16-QAM. Experimental results show that SEFDM with the proposed diversity technique performs robust against CD effects and outperforms the conventional OFDM with adaptive bit and power loading of the same bandwidth and data rate, which validates the superiority of the proposed SEFDM in optical IM/DD LR transmissions.

We investigate polarization mode dispersion (PMD) and polarization dependent loss (PDL) impairments in polarization division multiplexing (PDM) signals with optical polarization demultiplexing and direct detection. We find that the time alignment between the bits in the two polarizations has a significant impact on the PMD impairments, and PMD impairments also depend on the bandwidth of PDM signals, whereas PDL impairments have little dependence on the relative time alignment between the two polarizations and the signal bandwidth. We show that with a proper configuration of the polarization demultiplexing, the PDL-induced crosstalk between the two polarizations can be completely eliminated. The combined effects of PMD and PDL are also studied, and we find that, in the presence of concatenated PMD and PDL, the impairment from one effect does not enhance that from the other.

In this work, an improved receiving technique for Optical Wireless Intensity Modulation and Direct Detection (OW IM-DD) communication system, with Asymmetrically-Clipped Optical OFDM (ACO-OFDM) modulations, is proposed. The (socalled in the literature) "clipping noise", which carries 50% of the transmitted signal power is customarily discarded. Instead we show that the clipping noise carries information related to the original unclipped signal. Hence it is used in the proposed receiver scheme to generate an improved estimator for the transmitted signal. It is shown both theoretically and numerically, that the new approach improves the receiver performance of the system by up to 3dB without a change to the modulated signal.

The main objective of this paper is to achieve high normalized spectral efficiency (in bits per OCDMA chip) in optical CDMA network transmission. We design a time-wavelength M-ary transmission scheme that uses error-control coding and trellis coded modulation (TCM), optical M-ary intensity modulation and direct detection at the receiver. We use a probabilistic wavelength-time hopping approach, in order to transmit error-control coded and TCM modulated M-ary data directly through the optical channel without using optical spreading sequences. We present simulation results showing that the proposed OCDMA scheme is robust to Poisson and thermal noise. Hundreds of active users can be supported at target BER of 10-9 and achieved spectral efficiency (1.37 bits per OCDMA chip) is almost twice larger that the best result previously reported.

A four channel direct detection FDM operating at 565 Mbit/s with a channel spacing of 12 GHz which achieves a receiver sensitivity of −40 dBm is reported. A laser preamplifier is used to overcome the optical processing loss of the channel selective filter.

In this paper we introduce a nonlinear equalizer using the radial basis function (RBF) network with decision feedback equalizer (DFE) for electronic dispersion compensation in optical communication systems with on-off-keying and a direct detection receiver. The RBF method introduces a non-linear equalization technique suitable for optical communication direct detection systems that include nonlinear transformation at the photodetector. A bit error rate performance comparison shows that the RBF with DFE out performs the RBF without DFE and achieves similar results provided by maximum likelihood sequence estimator.

Time-frequency (TF) filtering of analog signals has played a crucial role in the development of radio-frequency communications and is currently being recognized as an essential capability for communications, both classical and quantum, in the optical frequency domain. How best to design optical time-frequency (TF) filters to pass a targeted temporal mode (TM), and to reject background (noise) photons in the TF detection window? The solution for 'coherent' TF filtering is known-the quantum pulse gate-whereas the conventional, more common method is implemented by a sequence of incoherent spectral filtering and temporal gating operations. To compare these two methods, we derive a general formalism for two-stage incoherent time-frequency filtering, finding expressions for signal pulse transmission efficiency, and for the ability to discriminate TMs, which allows the blocking of unwanted background light. We derive the tradeoff between efficiency and TM discrimination ability, and find a remarkably concise relation between these two quantities and the time-bandwidth product of the combined filters. We apply the formalism to two examples-rectangular filters or Gaussian filters-both of which have known orthogonal-function decompositions. The formalism can be applied to any state of light occupying the input temporal mode, e.g., 'classical' coherent-state signals or pulsed single-photon states of light. In contrast to the radio-frequency domain, where coherent detection is standard and one can use coherent matched filtering to reject noise, in the optical domain direct detection is optimal in a number of scenarios where the signal flux is extremely small. Our analysis shows how the insertion loss and SNR change when one uses incoherent optical filters to reject background noise, followed by direct detection, e.g. photon counting. We point out implications in classical and quantum optical communications. As an example, we study quantum key distribution, wherein strong rejection of background noise is necessary to maintain a high quality of entanglement, while high signal transmission is needed to ensure a useful key generation rate.

We present high-speed WDM-PON systems utilizing reflective semiconductor optical amplifiers and direct detection. We employ the polar RZ format and optical/electrical equalization techniques to maximize the performance of the highly band-limited devices.