Self-Similar Propagation and Amplification of Parabolic Pulses in Optical Fibers

Ultrashort pulse propagation in high gain optical fiber amplifiers with normal dispersion is studied by self-similarity analysis of the nonlinear Schrödinger equation with gain. An exact asymptotic solution is found, corresponding to a linearly chirped parabolic pulse which propagates self-similarly subject to simple scaling rules. The solution has been confirmed by numerical simulations and experiments studying propagation in a Yb-doped fiber amplifier. Additional experiments show that the pulses remain parabolic after propagation through standard single mode fiber with normal dispersion

Digital dual-rate burst-mode receiver for 10G and 1G coexistence in optical access networks

A digital dual-rate burst-mode receiver, intended to support 10 and 1 Gb/s coexistence in optical access networks, is proposed and experimentally characterized. The receiver employs a standard DC-coupled photoreceiver followed by a 20 GS/s digitizer and the detection of the packet presence and line-rate is implemented in the digital domain. A polyphase, 2 samples-per-bit digital signal processing algorithm is then used for efficient clock and data recovery of the 10/1.25 Gb/s packets. The receiver performance is characterized in terms of sensitivity and dynamic range under burst-mode operation for 10/1.25 Gb/s intensity modulated data in terms of both the packet error rate (PER) and the payload bit error rate (pBER). The impact of packet preamble lengths of 16, 32, 48, and 64 bits, at 10 Gb/s, on the receiver performance is investigated. We show that there is a trade-off between pBER and PER that is limited by electrical noise and digitizer clipping at low and high received powers, respectively, and that a 16/2-bit preamble at 10/1.25 Gb/s is sufficient to reliably detect packets at both line-rates over a burst-to-burst dynamic range of 14,5dB with a sensitivity of -18.5dBm at 10 Gb/s. (C)2011 Optical Society of Americ

Sparse Adaptive Frequency Domain Equalizers for Mode-Group Division Multiplexing

In this paper, we develop low complexity sparse frequency domain equalizers (FDEs) that exploit the sparsity that we observe in the graded index multimode fiber multiple-input multiple-output channel. The sparse channel impulse response is caused by the strong crosstalk at the mode MUX/DEMUX and weak coupling in the fiber between different mode groups. Two sparse FDE designs are proposed in order to compensate the crosstalk with relatively low computational complexity. The first method is based on a priori knowledge of the channel impulse response, which is used to generate a mask of taps with significant magnitudes. The second method is based on the improved proportionate normalized least-mean-square algorithm, where the active and inactive taps are adjusted at different rates of convergence. The computational complexity and the system performance of the proposed algorithms are analyzed. It is shown that the sparse FDEs offer low complexity relative to the sparse equalizers that use delay buffers, while maintaining improved performance over non-sparse equalizers in the presence of noise

Effect of catalyst pretreatment on chirality-selective growth of single-walled carbon nanotubes

We show that catalyst pre-treatment conditions can have a profound effect on the chiral distribution in single-walled carbon nanotubes chemical vapor deposition. Using a SiO2-supported Cobalt model catalyst and pre-treatment in NH3, we obtain a comparably narrowed chiral distribution with a downshifted tube diameter range, independent of the hydrocarbon source. Our findings demonstrate that the state of the catalyst at the point of nanotube nucleation is of fundamental importance for chiral control, thus identifying the pre-treatment atmosphere as a key parameter for control of diameter and chirality distributions.B.C.B acknowledges a Research Fellowship at Hughes Hall, Cambridge. J.R. thanks the Alexander von Humboldt Foundation for support.This is the original submitted version, prior to peer-review. The final version's available from ACS at http://pubs.acs.org/doi/abs/10.1021/jp4085348

Performance Comparison of Single-Sideband Direct Detection Nyquist-Subcarrier Modulation and OFDM

Direct detection transceivers offer advantages, including low cost and complexity, in short- and medium-haul links. We carried out studies seeking to identify the signal formats which offer the highest information spectral densities and maximum transmission distances for direct detection links. The performance of two spectrally efficient optical signal formats, single-sideband (SSB) Nyquist pulse-shaped subcarrier modulation (SCM) and SSB orthogonal frequency-division multiplexing (OFDM), are compared by means of simulations. The comparison is performed for a range of wavelength-division multiplexing (WDM) net information spectral densities up to 2.0 b/s/Hz by varying the signal bandwidth, modulation cardinality, and WDM channel spacing. The signal formats' tolerance to signal-signal beating interference, resulting from square-law detection, is investigated, and the Nyquist-SCM format is found to suffer lower penalties from this nonlinearity at high information spectral densities. In 7 × 28 Gb/s WDM transmission at 2.0 b/s/Hz (with electronic predistortion and EDFA-only amplification), Nyquist-SCM signals can be transmitted over distances of up to 720 km of standard SMF in comparison to a maximum of 320 km with the OFDM signal format

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 Nyquistsubcarrier modulation system operating at a net information spectral density of 2.3 (b/s)/Hz

Bidirectional symmetric 8x10. 7Gb/s WDM-PON over 108 km installed fiber using low complexity polarization-insensitive coherent ONUs

Polarization-time block-coded OFDM-QPSK downstream channels are robustly detected using a polarization-insensitive coherent receiver, consisting of only a 3dB coupler and single balanced PD. 8 x10.7Gb/s channels are bidirectionally transmitted over 108 km installed fiber achieving a 1:16-way passive split

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

Real time dynamic strain monitoring of optical links using the backreflection of live PSK data.

A major cause of faults in optical communication links is related to unintentional third party intrusions (normally related to civil/agricultural works) causing fiber breaks or cable damage. These intrusions could be anticipated and avoided by monitoring the dynamic strain recorded along the cable. In this work, a novel technique is proposed to implement real-time distributed strain sensing in parallel with an operating optical communication channel. The technique relies on monitoring the Rayleigh backscattered light from optical communication data transmitted using standard modulation formats. The system is treated as a phase-sensitive OTDR (ΦOTDR) using random and non-periodical non-return-to-zero (NRZ) phase-shift keying (PSK) pulse coding. An I/Q detection unit allows for a full (amplitude, phase and polarization) characterization of the backscattered optical signal, thus achieving a fully linear system in terms of ΦOTDR trace coding/decoding. The technique can be used with different modulation formats, and operation using 4 Gbaud single-polarization dual PSK and 4 Gbaud dual-polarization quadrature PSK is demonstrated. As a proof of concept, distributed sensing of dynamic strain with a sampling of 125 kHz and a spatial resolution of 2.5 cm (set by the bit size) over 500 m is demonstrated for applied sinusoidal strain signals of 500 Hz. The limitations and possibilities for improvement of the technique are also discussed.This work was supported by the European Research Council through Starting Grant UFINE (Grant no. 307441), the Spanish MINECO through project TEC2013-45265-R, PCIN-2015- 219, and the regional program SINFOTON-CM: S2013/MIT-2790. HFM acknowledges EU funding through the FP7 ITN ICONE program, gr. #608099. SML acknowledges funding from the Spanish MINECO through a “Ramon y Cajal” contract. UK EPSRC funding through project EP/J008842/1

Comparison of low complexity coherent receivers for UDWDM-PONs (λ-to-the-User)

It is predicted that demand in future optical access networks will reach multigigabit/s per user. However, the limited performance of the direct detection receiver technology currently used in the optical network units at the customers' premises restricts data rates per user. Therefore, the concept of coherent-enabled access networks has attracted attention in recent years, as this technology offers high receiver sensitivity, inherent frequency selectivity, and linear field detection enabling the full compensation of linear channel impairments. However, the complexity of conventional (dual-polarization digital) coherent receivers has so far prevented their introduction into access networks. Thus, to exploit the benefits of coherent technology in access networks, low complexity coherent receivers, suitable for implementation in ONUs, are needed. In this paper, the recently proposed low complexity coherent (i.e., polarization-independent Alamouti-coding heterodyne) receiver is, for the first time, compared in terms of its minimum receiver sensitivity with five previously reported receiver designs, including a detailed discussion on their advantages and limitations. It is shown that, of all the configurations considered, the Alamouti-coding based receiver approach allows the lowest number of photons per bit (PPB) transmitted (with a lower bound of 15.5 PPB in an ideal implementation of the system), while requiring the lowest optical receiver hardware complexity (in terms of the optical component count). It also exhibits comparable complexity to the currently deployed direct-detection receivers, which typically require over 1000 PPB. Finally, a comparison of experimentally achieved receiver sensitivities and transmission distances using these receivers is presented. The highest spectral efficiency and longest transmission distance at the highest bit rate (10 Gb/s) was reported using the Alamouti-coding receiver, which is also the only one, to date, to have been demonstrated in a full system bidirectional transmission