Optical Time Domain Multiplexing Research Articles

Eigen-function division multiplexed coherent optical transmission in time domain by using higher-order Hermite-Gaussian pulses.

We describe a new multiplexing technique and its application to demultiplexing in the time domain by using higher-order Hermite-Gaussian (HG) pulses, which are solutions of the Schrödinger equation. We call this technique eigen-function division multiplexing (EDM). This method enables us to further increase the total transmission capacity by superimposing many different HG pulses in the same time slot. This technique is different from a conventional optical time domain multiplexing (OTDM) technique using interleaving, where one pulse exists only in one time slot. The transmitted EDM HG pulses can be demultiplexed by adopting the time-domain orthogonality of the HG pulses (eigen-function orthogonality). The information carried by the mth-order HG pulse (HGm pulse) can be coherently detected by a photo detector, where photo-mixing with a phase-locked HGm pulse generated by a local oscillator can realize demultiplexing. The overlap integral with a different HG pulse becomes zero due to the time domain orthogonality. First, we show numerically that such a new EDM transmission scheme in the time domain is possible. We then show experimentally that we could successfully carry out an EDM HG coherent pulse transmission with four different HG pulses (HG0, HG1, HG2, and HG3), where we report a 400∼480 Gbit/s (10 Gbaud x 4 eigen-functions x 2 pol-mux.) 32∼64 QAM EDM transmission over 300∼450 km.

Nonlinear feedforward enabling quantum computation

Measurement-based quantum computation with optical time-domain multiplexing is a promising method to realize a quantum computer from the viewpoint of scalability. Fault tolerance and universality are also realizable by preparing appropriate resource quantum states and electro-optical feedforward that is altered based on measurement results. While linear feedforward has been realized and become a common experimental technique, nonlinear feedforward was unrealized until now. In this paper, we demonstrate that a fast and flexible nonlinear feedforward realizes the essential measurement required for fault-tolerant and universal quantum computation. Using non-Gaussian ancillary states, we observed 10% reduction of the measurement excess noise relative to classical vacuum ancilla.

Monocular metasurface camera for passive single-shot 4D imaging

It is a grand challenge for an imaging system to simultaneously obtain multi-dimensional light field information, such as depth and polarization, of a scene for the accurate perception of the physical world. However, such a task would conventionally require bulky optical components, time-domain multiplexing, and active laser illumination. Here, we experimentally demonstrate a compact monocular camera equipped with a single-layer metalens that can capture a 4D image, including 2D all-in-focus intensity, depth, and polarization of a target scene in a single shot under ambient illumination conditions. The metalens is optimized to have a conjugate pair of polarization-decoupled rotating single-helix point-spread functions that are strongly dependent on the depth of the target object. Combined with a straightforward, physically interpretable image retrieval algorithm, the camera can simultaneously perform high-accuracy depth sensing and high-fidelity polarization imaging over an extended depth of field for both static and dynamic scenes in both indoor and outdoor environments. Such a compact multi-dimensional imaging system could enable new applications in diverse areas ranging from machine vision to microscopy.

A comparative study of chromatic dispersion compensation in 10 Gbps SMF and 40 Gbps OTDM systems using a cascaded Gaussian linear apodized chirped fibre Bragg grating design

Optical time domain multiplexed (OTDM) technology helps to respond to the need for increased capacity and demand in internet traffic using optical transmissions. High-speed optical networks are highly affected by chromatic dispersion (CD) that causes pulse broadening and intersymbol interference (ISI) at the light detector. This paper presents a comparative analysis of chromatic dispersion compensation on 10 Gbps single mode fibre (SMF) and 40 Gbps OTDM operating at C-band using Gaussian apodized linear chirped fibre Bragg grating, Erbium Doped Fibre Amplifier (EDFA) and uniform fibre Bragg grating (UFBG) hybrid approach in OptiSystem 18. A cascade of four UFBG and chirped Bragg gratings design approach presents minimal improvement in the quality factor and achievable single mode fibre length. Quality factor results at a reference wavelength of 1550 nm were obtained for respective chirped Bragg grating lengths of 8 mm, 20 mm, 50 mm, 87 mm, and 90 mm using linear Gaussian apodization. The Q-factor results obtained using the proposed simulation model for single 10 Gb/s channel were respectively 51.14 dBm over 30 km at a grating length of 20 mm and 15.64 dBm over 150 km at 45 mm of grating length. Different eye-diagrams corresponding to varying grating lengths have also been presented for 10 Gb/s and 40 Gb/s at corresponding SMF lengths.

Multifunctional Wavelength-Selective Switch for Nyquist Pulse Generation and Multiplexing

We demonstrate optical Nyquist pulse train generation using a wavelength-selective switch (WSS) without an optical multiplexing circuit. The representative approach generating optical Nyquist pulse trains is based on spectral filtering with a WSS and optical time-domain multiplexing (OTDM) with an optical circuit. The use of the optical circuit causes difficulties when extending optical Nyquist pulse train technology to the near-infrared band because this band has no well-established OTDM modules. To resolve this problem, we assign the role of multiplexing to the WSS, in addition to spectral filtering. Furthermore, the WSS-only approach is expected to reduce optical losses and increase pulse-train parameter flexibility. Using only the WSS, we successfully generate optical Nyquist pulse trains in the near-infrared band and realize flexible control of pulse number, width, and interval. In the case of a 10-pulse train, optical loss due to multiplexing is improved by 8.0 dB compared with the conventional method.

A High-Speed Second-Order Photonic Differentiator Based on Two-Stage Silicon Self-Coupled Optical Waveguide

In this paper, we propose and demonstrate an all-optical second-order differentiator based on a two-stage self-coupled optical waveguide on a silicon-on-insulator platform. The transmission spectrum of the fabricated device has a parabola-like filtering notch with a 3-dB bandwidth of up to ~ 100 GHz and a depth of ~ 12 dB. Experiments are carried out for 10-, 20-, and 40-Gb/s optical time-domain multiplexing (OTDM) picosecond pulse trains, and the second-order differentiations are achieved using the fabricated device.

NUMERICAL ANALYSIS OF THE INSTANTANEOUS AND RELAXED KERR MODEL FOR GENERATION OF THE ALL-OPTICAL LOGIC GATES WITH TRIANGULAR FIBER COUPLER (TFC)

All-optical logic gates can enable many advanced functions such as all-optical bit-pattern recognition, all-optical bit-error rate monitoring, all-optical packet address and payload separation, all-optical label swapping and all-optical packet drop in optical time domain multiplexing (OTDM) networks. Recently, much attention has been given to the influence of the relaxation process (sometimes called the Debye relaxation model) of the nonlinear response because the usual assumption of instantaneous nonlinear response fails for ultrashort pulses and additional contributions coming from nonlinear dispersion and relaxed nonlinearity have to be taken into account. The Kerr–Debye model is a relaxation of the nonlinear Kerr model in which the relaxation coefficient is a finite response time of the nonlinear material. In this paper, we have presented a numerical analysis of the triangular fiber coupler (TFC) for generation of the all-optical logic gates with nonlinear optical (NLO) properties, where we consider the nonlinear effects Kerr group velocity dispersion (GVD) and self-phase modulation (SPM) instantaneous and relaxed (Kerr–Debye model). To implement all-optical logic gates we used TFC of three symmetric configurations [Instantaneous (III), Relaxed (RRR-5 and RRR-9)]. In the instantaneous condition, the TFC is made up of silica optical fibers (with instantaneous response time — indicated by III) and in the relaxed conditions (RRR-5 and RRR-9) the TFC is made up of fibers with delayed response time of around 25 ps (for example, the polymer optical fibers). In our paper, we are interested in the transmission characteristics, the XRatio level (XR (dB)) as a function of the ΔΦ parameter, the normalized time duration (NTD) and the pulse evolution along the TFC and finally to compare the performance of all-optical logic gates, we will use the figure-of-merit of the logic gates (FOMELG (dB)) defined as a function of the extinction ratio of the gate outputs. All results were obtained numerically, considering a very simple model for generation of a optical logic gates.

Self-clocked 80 Gbits/s optical time-domain multiplexing transmission with clock distribution based on amplitude discrimination

Conventional all-optical feedback-based clock recovery techniques for optical time-domain multiplexing (OTDM) networks place restrictions on the allowed data patterns that can be transmitted. We propose a data-independent clock distribution solution based on amplitude discrimination and experimentally demonstrate it in an 80 Gbits/s self-clocked OTDM transmission. According to the method a single OTDM subchannel is used for exchanging clock information. All processing is performed all optically in low latency nonlinear-optical-loop-mirror-based switches with short (approximately 10 m) nonlinear elements.

Polarization independent 4-stage OTDM multiplexer using plated GRIN lens

In this paper, a 4-stage optical time domain multiplexing (OTDM) multiplexer based on plated graded index (GRIN) lens is proposed and experimentally demonstrated. A 10Gbit/s return-to-zero (RZ) signal is upgraded to 160Gbit/s. The time-domain accuracy of the multiplexer is evaluated by analyzing the multiplexed 160Gbit/s signal. Experimental results validate the stability of the optical multiplexing behavior and the polarization insensitivity of the proposed multiplexer. The results also show the advantages of our OTDM multiplexer such as the flexible output signal speeds at the output of different stages, the low insertion loss and the temperature and wavelength stability.

Tolerances and Receiver Sensitivity Penalties of Multibit Delay Differential-Phase Shift-Keying Demodulation

Multibit delay demodulation of differential-phase shift-keying (DPSK) is finding applications in polarization interleaved modulation, optical time-domain multiplexing (OTDM), and multisymbol DPSK demodulation. Little attention has been paid to the degradation in tolerance and power penalty associated with multibit delay demodulation. We assess experimentally, numerically, and analytically the power penalties and tolerances associated with multibit delay DPSK demodulation. Numerical and analytical results show that the power penalty scales by a small factor of 0.2-0.35 dB per integer bit delay due to laser linewidth (LW) while experimental back-to-back results show a significant 1.2 dB per integer bit delay due to frequency offset penalty of longer bit delays. Frequency offset tolerance scales as 1/bit-delay and the delay-mismatch tolerance decreases by 20% for delays longer than 1 bit. A simple analytic model accounts for the combined effect of LW, frequency offset, and amplified spontaneous emission.

Time-domain polarization interleaving of signal to allow polarization-insensitive all-optical clock recovery

This letter presents a novel scheme for polarization-insensitive optical clock recovery by preconditioning the data signal at the transmitter side. The polarization-insensitivity is realized by making the polarizations of two channels orthogonal in the second stage of optical time-domain multiplexing. A 38-GHz optical clock was recovered from the incoming data stream using a passively mode-locked multisection laser diode. The results show that the performance of the recovered 38-GHz clock is extremely insensitive to the polarization state of the incoming data.

Error-free all-optical add-drop multiplexing using HNLF in a NOLM at 160 Gbit/s

The capability of a highly-nonlinear fibre (HNLF) as the phase shifting element in a nonlinear optical loop mirror (NOLM) for all-optical adding/dropping optical time domain multiplexed (OTDM) channels has been demonstrated. Error-free add/drop operation of 40 Gbit/s base rate channels from a 160 Gbit/s OTDM signal has been achieved.

Nonlinear Optical Loop Mirrors: Investigation Solution and Experimental Validation for Undesirable Counterpropagating Effects in All-Optical Signal Processing

The ultrashort response time of the Kerr effect in the optical fiber suggests the possibility of using nonlinear optical loop mirrors (NOLMs) in applications of ultrafast all-optical signal processing. Roughly speaking, only one of the signal halves in the fiber loop undergoes a phase shift, due to its own power [self-phase modulation (SPM)] or to a copropagating pump light [cross-phase modulation (XPM)]. But even the other signal half is affected by SPM or XPM induced by the mean power of the strong counterpropagating signal. When considering ultrashort pulse trains at low bit rate, the phase shift due to the mean power can be neglected. But as the repetition rate of the pulse train gets higher, as in an optical time-domain multiplexing) frame, the effect of counterpropagating power must be taken into account, as it tends to reduce the efficiency of the NOLM. Up to now, some schemes have been proposed in the literature to eliminate these limitations, but they usually add further complexity. In this paper, we will investigate the impact of the undesirable effects due to the counterpropagating power on the NOLM performance for different duty-cycle values. We will present a simple and low-cost solution to overcome these impairments for both SPM- and XPM-based NOLMs. The solution consists of a proper NOLM design, using non-polarization maintaining) fiber and including a polarization controller into the loop, in order to compensate for the counterpropagating effects. Finally, the proposed solution will be experimentally validated.

Simultaneous high speed OTDM add-drop multiplexing using GT-UNI switch

The authors describe the excellent capability of an all-optical gain-transparent ultrafast nonlinear interferometer (GT-UNI) in dropping, passing through, and adding optical time domain multiplexing (OTDM) channels. Error free operation without significant penalties of a complete OTDM add-drop node at 80 Gbit/s was achieved.

1.28 Tbit/s—70‐km OTDM femtosecond‐pulse transmission using third‐ and fourth‐order simultaneous dispersion compensation with a phase modulator

AbstractBy means of the higher‐order dispersion compensation technique using a phase modulator, a 70‐km transmission experiment with 1.28 Tbit/s OTDM (Optical Time Domain Multiplexing) signals has been successfully conducted for the first time. The details are reported in this paper. The 1.28 Tbit/s signal is obtained by 64‐fold multiplexing of 10 Gbit/s signals with additional polarization multiplexing. For the signal pulse with a pulse width of 380 fs prior to transmission, a linear chirp is imparted and then a cosine phase modulation with an appropriate amplitude and the timing is set in such a way that the imparted chirp is opposite to the chirps caused by the third‐ and fourth‐order dispersion in the transmission path. In this way, the waveform distortion of the signal due to the third‐ and fourth‐order dispersion can be suppressed simultaneously. The pulse widening of the signal after transmission over 70 km is only 20 fs. Hence, an error rate of less than 1 × 10−9 can be realized over the entire channel (10 Gbit/s–128 channels after multiplexing separation). © 2002 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 86(3): 68–79, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecja.10028

40-Gb/s OTDM to 4 x 10 Gb/s WDM conversion in monolithic InP Mach-Zehnder interferometer module

Transformation of high bit-rate optical time-domain multiplexed (OTDM) signals into a multitude of lower bit-rate wavelength-division-multiplexed (WDM) channels is demonstrated by means of a single monolithically integrated indium phosphide Mach-Zehnder interferometer with semiconductor optical amplifiers in its arms. Full demultiplexing of 10-Gb/s OTDM signals into 4×10-Gb/s WDM channels is demonstrated. Bit-error-rate penalties are below 1.5 dB for polarization independent signal conversion throughout the 1.55-μm wavelength range.

Multiple output semiconductor ring lasers with high external quantum efficiency

The external quantum efficiency of semiconductor ring lasers has been significantly improved by introducing output couplers that couple more light out of the ring cavity. Improvements from 10% and 16% to 14% and 27% of a comparable single 2×2 MMI coupler device were achieved by using 3×3 multimode interference (MMI) couplers and dual 2×2 MMI couplers, respectively. A further increase to the value of 33% (or 2 times that of the previous devices) was obtained by AR coating the output facets of the dual 2×2 MMI coupler devices. These devices also provided four output beams as required in an integrated optical time domain multiplexing (OTDM) system.

Are crossbars really dead?

Crossbar switches are rarely considered for large, scalable multiprocessor interconnect systems because they require O(n 2 ) switching elements, are difficult to control efficiently and are hard to implement once their size becomes too large to fit on one integrated circuit. However these problems are technology dependent and a recent innovation in fiber optic devices has led to a new implementation of crossbar switches that does not share these problems while retaining the full advantages of a crossbar switch: low latency, high throughput, complete connectivity and multi-cast capability. Moreover, this new technology has several characteristics that allow a distributed control system which scales linearly in the number of attached nodes.The innovation that led to this research is an optical and-gate that can be used to demultiplex multiple high speed data streams that are carried on one common optical medium. Optical time domain multiplexing can combine the data from many nodes and broadcast the result back to all nodes. This paper discusses OTDM technology only to the extent necessary to understand its characteristics and capabilities. The main contribution lies in the description and analysis of interconnect architectures that utilize OTDM to achieve a level performance that is beyond electronic means. It is expected that cost-reduced OTDM systems will become competitive with the next generation of interconnect systems.