M. Sezer Erkilinc

Signal-signal beat interference cancellation in spectrally-efficient WDM direct-detection Nyquist-pulse-shaped 16-QAM subcarrier modulation

An experimental demonstration of direct-detection single-sideband Nyquist-pulse-shaped 16-QAM subcarrier modulated (Nyquist-SCM) transmission implementing a receiver-based signal-signal beat interference (SSBI) cancellation technique is described. The performance improvement with SSBI mitigation, which compensates for the nonlinear distortion caused by square-law detection, was quantified by simulations and experiments for a 7 × 25 Gb/s WDM Nyquist-SCM signal with a net optical information spectral density (ISD) of 2.0 (b/s)/Hz. A reduction of 3.6 dB in the back-to-back required OSNR at the HD-FEC threshold was achieved. The resulting reductions in BER in single channel and WDM transmission over distances of up to 800 km of uncompensated standard single-mode fiber (SSMF) achieved are presented.

Spectrally Efficient WDM Nyquist Pulse-Shaped 16-QAM Subcarrier Modulation Transmission With Direct Detection

The ability to transmit signals with high information spectral density (ISD) using low-complexity and cost-effective transceivers is essential for short- and medium-haul optical communication systems. Consequently, spectrally efficient direct detection transceiver-based solutions are attractive for such applications. In this paper, we experimentally demonstrate the wavelength-division multiplexed (WDM) transmission of 7×12 GHz-spaced dispersion pre-compensated Nyquist pulse-shaped 16-QAM subcarrier modulated channels operating at a net bit rate of 24 Gb/s per channel, and achieving a net optical ISD of 2.0 b/s/Hz. The direct detection receiver used in our experiment consisted of a single-ended photodiode and a single analog-to-digital converter. The carrier-to-signal power ratio at different values of optical signal-to-noise ratio was optimized to maximize the receiver sensitivity performance. The transmission experiments were carried out using a recirculating fiber loop with uncompensated standard single-mode fiber and EDFA-only amplification. The maximum achieved transmission distances for single channel and WDM signals were 727 and 323 km below the bit-error ratio of 3.8 × 10-3, respectively. To the best of our knowledge, this is the highest achieved ISD for WDM transmission in direct detection links over such distances.

SSBI Mitigation and the Kramers–Kronig Scheme in Single-Sideband Direct-Detection Transmission With Receiver-Based Electronic Dispersion Compensation

The performance of direct-detection transceivers employing electronic dispersion compensation combined with DSP-based receiver linearization techniques is assessed through experiments on a 4 × 112 Gb/s wavelength-division multiplexing direct-detection single-sideband 16 quadratic-amplitude modulation Nyquist-subcarrier-modulation system operating at a net optical information spectral density of 2.8 b/s/Hz in transmission over standard single mode fiber links of up to 240 km. The experimental results indicate that systems with receiver-based dispersion compensation can achieve similar performance to those utilizing transmitter-based dispersion compensation, provided it is implemented together with an effective digital receiver linearization technique. The use of receiver-based compensation would simplify the operation of a fiber link since knowledge of the link dispersion is not required at the transmitter. The recently proposed Kramers–Kronig receiver scheme was found to be the best performing among the receiver linearization techniques assessed. To the best of our knowledge, this is the first experimental demonstration of the Kramers–Kronig scheme.

Simplified DSP-Based Signal-Signal Beat Interference Mitigation Technique for Direct Detection OFDM

Short- and medium-haul links in interdata center, access, and metro networks require cost-effective direct-detection wavelength-division multiplexing transceivers offering energy efficiency, high information spectral density (ISD), and dispersion tolerance. Single-sideband orthogonal frequency-division multiplexing (SSB-OFDM) with direct detection is a potential solution; however, it suffers a penalty from signal-signal beat interference (SSBI) caused by the square-law photodetection. In this paper, a novel DSP-based SSBI mitigation technique, with lower complexity than previously proposed methods, is proposed and assessed through numerical simulations for the first time. The performance improvement is quantified by simulations of 9 × 112 Gb/s 16-QAM SSB-OFDM signal with a net optical ISD of 2.1 (b/s)/Hz. The performance is shown to be similar to that of the more complex receiver-based iterative SSBI compensation technique. Simulations predict an 8.7 dB reduction in the required OSNR at the 7% overhead HD-FEC threshold, and increases up to 100% in maximum reach over uncompensated standard single-mode fibre using the proposed simplified SSBI compensation technique.

Comparison of digital signal-signal beat interference compensation techniques in direct-detection subcarrier modulation systems

Single-polarization direct-detection transceivers may offer advantages compared to digital coherent technology for some metro, back-haul, access and inter-data center applications since they offer low-cost and complexity solutions. However, a direct-detection receiver introduces nonlinearity upon photo detection, since it is a square-law device, which results in signal distortion due to signal-signal beat interference (SSBI). Consequently, it is desirable to develop effective and low-cost SSBI compensation techniques to improve the performance of such transceivers. In this paper, we compare the performance of a number of recently proposed digital signal processing-based SSBI compensation schemes, including the use of single- and two-stage linearization filters, an iterative linearization filter and a SSBI estimation and cancellation technique. Their performance is assessed experimentally using a 7 × 25 Gb/s wavelength division multiplexed (WDM) single-sideband 16-QAM Nyquist-subcarrier modulation system operating at a net information spectral density of 2.3 (b/s)/Hz.

Polarization-Insensitive Single-Balanced Photodiode Coherent Receiver for Long-Reach WDM-PON s

In an access network based on a passive optical network architecture, coherent detection is attractive since it allows for high receiver sensitivity coupled with inherent frequency selectivity. Nevertheless, solutions employed in core networks are prohibitively complex and costly, requiring the optical complexity of the coherent receivers to be reduced to make them feasible for access networks. For monolithic integration, a key challenge is posed by the polarization beam splitter (PBS). If, however, the PBS is removed, the receiver needs to be redesigned to be insensitive to the incoming polarization state of the received signal. In this paper, we experimentally demonstrate a polarization-insensitive (i.e., polarization-independent) coherent receiver for the optical network unit in passive optical networks (PONs). The receiver consists of only a 3-dB coupler and a single-balanced photodiode such that the complexity is comparable to a direct detection receiver. The proposed cost-effective coherent receiver is implemented by using the Alamouti polarization-time block coding scheme combined with heterodyne detection. To verify the technique, the Alamouti-coded orthogonal frequency division multiplexing (OFDM) signal is rotated over the full Poincaré sphere. Compared to the dual-polarization-OFDM signal operating at a net bit rate of 10 Gb/s per polarization (a gross bit rate of 10.7 Gb/s including a 7% FEC overhead), only a 0.6 dB sensitivity degradation is observed. The sensitivity at the FEC threshold, assumed to be 4×10-3, is measured to be -41.5 dBm (56 photons-per-bit) on a 25-GHz grid. Following this, different channel spacings are investigated and the signal is transmitted over 80 km of standard single-mode fiber in a long-reach wavelength division multiplexed PON system. The loss budgets are found to be 43.0 and 42.8 dB for 50- and 25-GHz grids, respectively.

Spectrally Efficient WDM Nyquist Pulse-Shaped Subcarrier Modulation Using a Dual-Drive Mach–Zehnder Modulator and Direct Detection

High data transmission capacity is increasingly needed in shortand medium-haul optical communication links. Cost-effective wavelength division multiplexed (WDM) transceiver architectures, achieving high information spectral densities (ISDs) (>1 b/s/Hz) and using low-complexity direct detection receivers are attractive solutions for such links. In this paper, we assess the use of dual-drive Mach-Zehnder modulators (DD-MZMs), and compare them with in-phase quadrature (IQ)-modulators for generating spectrally-efficient single sideband Nyquist pulse-shaped 16-QAM subcarrier (N-SCM) modulation format signals. The impact of the extinction ratio (ER) of a modulator on the optical sideband suppression ratio (OSSR) was investigated for the SSB signals in WDM systems, together with the resulting impact on inter-channel crosstalk penalties. First, in back-to-back operation, an IQ-modulator with an ER of 30 dB and a DD-MZM with an ER of 18 dB were experimentally compared in a 6×25 Gb/s WDM system by varying the channel spacing. Following this comparison, 16 GHz-spaced 6×25 Gb/s WDM signal transmission was experimentally demonstrated using the DD-MZM. The experiment was performed using a recirculating loop with uncompensated standard single-mode fiber (SSMF) and EDFA-only amplification. The maximum achievable transmission distances for single channel and WDM signals were found to be 565 and 242 km, respectively, at a net optical ISD of 1.5 b/s/Hz. This is the first experimental comparison of such modulator types for SSB N-SCM signal generation and the highest achieved ISD using a DD-MZM in direct-detection WDM transmission.

Text, photo, and line extraction in scanned documents

Abstract. We propose a page layout analysis algorithm to classify a scanned document into different regions such as text, photo, or strong lines. The proposed scheme consists of five modules. The first module performs several image preprocessing techniques such as image scaling, filtering, color space conversion, and gamma correction to enhance the scanned image quality and reduce the computation time in later stages. Text detection is applied in the second module wherein wavelet transform and run-length encoding are employed to generate and validate text regions, respectively. The third module uses a Markov random field based block-wise segmentation that employs a basis vector projection technique with maximum a posteriori probability optimization to detect photo regions. In the fourth module, methods for edge detection, edge linking, line-segment fitting, and Hough transform are utilized to detect strong edges and lines. In the last module, the resultant text, photo, and edge maps are combined to generate a page layout map using K-Means clustering. The proposed algorithm has been tested on several hundred documents that contain simple and complex page layout structures and contents such as articles, magazines, business cards, dictionaries, and newsletters, and compared against state-of-the-art page-segmentation techniques with benchmark performance. The results indicate that our methodology achieves an average of ∼89% classification accuracy in text, photo, and background regions.

Two-Stage Linearization Filter for Direct-Detection Subcarrier Modulation

A novel digital two-stage linearization filter is proposed for direct-detection (DD) systems and assessed experimentally for the first time. The performance improvement is quantified by experiments with a 7 × 25 Gb/s wavelength division multiplexing DD single sideband 16-QAM Nyquist-shaped subcarrier modulation system with a net optical information spectral density of 2.3 (b/s)/Hz. The results indicate that this technique can effectively compensate the nonlinearity caused by square-law detection, while, at the same time reducing the DSP complexity, avoiding the need to perform the multiple iterations which are required in previously proposed beating interference cancellation methods.

Dispersion-precompensated direct-detection nyquist pulse-shaped subcarrier modulation using a dual-drive Mach-Zehnder modulator

We report the experimental demonstrations of direct detection 12.5/25 Gb/s single sideband Nyquist pulse-shaped QPSK/16-QAM subcarrier modulation, generated using a dual-drive Mach-Zehnder modulator. Utilizing electronic pre-distortion, transmission over 1614/484km of standard SMF was achieved.