Optical Time Division Multiplexing Signal Research Articles

Free-space transmission of picosecond-level, high-speed optical pulse streams in the 3 µm band.

The utilization of mid-infrared (mid-IR) light spanning the 3-5 µm range presents notable merits over the 1.5 µm band when operating in adverse atmospheric conditions. Consequently, it emerges as a promising prospect for serving as optical carriers in free-space communication (FSO) through atmospheric channels. However, due to the insufficient performance level of devices in the mid-IR band, the capability of mid-IR communication is hindered in terms of transmission capacity and signal format. In this study, we conduct experimental investigations on the transmission of time-domain multiplexed ultra-short optical pulse streams, with a pulse width of 1.8 ps and a data rate of up to 40 Gbps at 3.6 µm, based on the difference frequency generation (DFG) effect. The mid-IR transmitter realizes an effective wavelength conversion of optical time division multiplexing (OTDM) signals from 1.5 µm to 3.6 µm, and the obtained power of the 40 Gbps mid-IR OTDM signal at the optimum temperature of 54.8 °C is 7.4 dBm. The mid-IR receiver successfully achieves the regeneration of the 40 Gbps 1.5 µm OTDM signal, and the corresponding regenerated power at the optimum temperature of 51.5 °C is -30.56 dBm. Detailed results pertaining to the demodulation of regeneration 1.5 µm OTDM signal have been acquired, encompassing parameters such as pulse waveform diagram, bit error rate (BER), and Q factor. The estimated power penalty of the 40 Gbps mid-IR OTDM transmission is 2.4 dB at a BER of 1E-6, compared with the back-to-back (BTB) transmission. Moreover, it is feasible by using chirped PPLN crystals with wider bandwidth to increase the data rate to the order of one hundred gigabits.

Dispersion-tolerant 200-Gb/s OTDM-PAM4 system using a simple phase-alternating pulse generator

We propose to implement a simple 2-channel optical-time-division multiplexed (OTDM) system based on a phase-alternating pulse generator for a short-reach optical interconnects. The inserted pulse generator consisting of a phase modulator and an optical periodic filter can generate the optical pulse of which the repetition rate is double the frequency of the input electrical clock. The phase-alternating property of the pulse makes the OTDM signal robust to the chromatic dispersion. In the experiment, we optimize the optical pulse, use it for generating the 200-Gb/s OTDM signal operating in the 4-level pulse-amplitude modulated format, and transmit the signal over the 1.9-km standard single-mode fiber.

A Cost-Effective 2-Channel OTDM System Implemented With Sinusoidally Modulated Light Source

We propose to implement a 2-channel optical-time-division-multiplexed (OTDM) system for short-reach optical interconnects by using a sinusoidally modulated light source instead of a complicated mode-locked laser as an input pulse source. In this system, the OTDM signal is obtained by bit-interleaving two optical return-to-zero (RZ) signals generated by using the sinusoidally modulated light. We operate these RZ signals in the orthogonal in-phase and quadrature domains to avoid the unwanted beat components. After the transmission, the OTDM signal is detected by using single photodetector, and then processed by a 2 × 2 multiple-input multiple-output equalizer. For a demonstration, we generate 150-Gb/s OTDM signal operating in the 8-level pulse-amplitude modulated (PAM-8) format by using commercial LiNbO 3 Mach-Zehnder modulators and transmit this OTDM signal over 1.9 km of the standard single-mode fiber (SSMF). In addition, we fabricate the proposed OTDM transmitter in an integrated silicon-photonics chip and use it to demonstrate the transmission of the 64-Gb/s OTDM PAM-4 signal over 2.2 km of SSMF.

Slot-slot waveguide with negative large and flat dispersion covering C+L+U waveband for on-chip photonic networks.

A novel, to the best of our knowledge, dual-core slot-slot waveguide with extreme high dispersion is proposed. The high dispersion value at the desired wavelength is obtained based on strong resonance coupling between two slot-waveguide modes. The properties of dispersion magnitude and bandwidth are numerically analyzed by using the finite-difference time-domain method with a perfectly matched layer boundary. All numerical simulation results reveal that for the optimized geometrical parameters of H1=350 nm, L1=569 nm, S1=31.3 nm, La=1062.39 nm, H2=427 nm, L2=137.4 nm, and S2=63.5 nm, the maximum dispersion of negative 3.645×105 ps·nm-1·km-1 and dispersion full width at half-maximum of 6.3nm at 1550nm have been obtained. By cascading the slot-slot waveguides with varying width and height, a large and flattened dispersion of -3.5×105 ps·nm-1·km-1 covering the C+L+U waveband is obtained. Dispersion compensation of a 100Gbit/s return-to-zero on-off-keying optical time-division multiplexing signal after 50km full spectrum single-mode optical fiber transmission with five different central wavelengths is demonstrated through simulation for the first time. In addition, fabrication tolerance of the proposed slot-slot waveguide is analyzed. Such a waveguide is compatible with complementary metal-oxide-semiconductor technology and has potential applications in next-generation large-scale photonic integrated circuits.

All-optical demultiplexing of Nyquist OTDM signal using a biorthogonal Nyquist gate

We present an all-optical method for demultiplexing Nyquist optical time division multiplexed (OTDM) signals using a biorthogonal Nyquist gate, which is a Nyquist gate of reduced pulse width. The biorthogonality between the Nyquist symbol and Nyquist gate creates an intersymbol interference (ISI) free spectral region in the mixed product that theoretically allows for complete elimination of ISI in the demultiplexer. Furthermore, this format transparent all-optical method produces a Nyquist signal on the output suitable for further optical processing and multiplexing. We analyze the performance of this approach through simulation in the presence of nonidealities of imperfect pulse shape, timing jitter, and dispersion. Finally, we experimentally demonstrate the performance of this approach operating on an 80-GBd Nyquist OTDM signal.

Simultaneous wavelength exchange and 2R regeneration for two optical time division multiplexing signals by a single highly nonlinear fiber

The simultaneous wavelength exchange and 2R regeneration between two degraded optical time division multiplexing (OTDM) signals is experimentally demonstrated. By exploiting the self-phase modulation effect and offset filtering in a single highly nonlinear fiber (HNLF), it achieves 2R optical regeneration. Meanwhile, it realizes wavelength exchange using a bidirectional configuration. Optical wavelength exchange and 2R of 20- and 50-Gbit/s OOK signals at 1557.3 and 1552.3 nm is implemented with a power penalty of ∼0.5 dB at a bit-error rate of 10−9. The results show that the crosstalk is negligible for both channels in opposite directions. This bidirectional configuration exhibits excellent performance for simultaneous wavelength exchange and 2R regeneration between two degraded OTDM signals with different wavelengths.

All-Optical Bit-Rate Flexible NRZ-to-RZ Conversion Using an SOA-Loop Mirror and a CW Holding Beam

All-optical non-return-to-zero (NRZ) -to- return-to-zero (RZ) data-format conversion has been successfully demonstrated using a semiconductor optical amplifier in a fiber-loop mirror (so-called SOA-loop mirror) with a continuous-wave (CW) holding beam. The converted RZ signal after pulse compression has been used to create a 40 Gb/s OTDM (Optical Time Division Multiplexing) signal. Here is proposed an NRZ-to-RZ conversion method without any additional optical clocks, unlike conventional methods based on optical AND logic. In addition, it has the merit of operating at various bit-rate speeds without any controlling device. Moreover, it has a simple structure, and it can be used for all-optical bit-rate-flexible clock recovery.

Combined Optical and Electrical Spectrum Shaping for High-Baud-Rate Nyquist-WDM Transceivers

We discuss the benefits and limitations of optical time-division multiplexing (OTDM) techniques based on the optical generation of a periodic train of sinc pulses for wavelength-division multiplexing (WDM) transmission at high baud rates. It is shown how the modulated OTDM spectrum bandwidth is related to the optical comb parameters and the pulse shaping of the modulating waveforms in the electrical domain. Such dependence may result in broadening of the modulated spectra, which can degrade the performance of Nyquist-WDM systems due to interchannel crosstalk penalties. However, it is shown and experimentally demonstrated that the same technique of optical pulse train generation can be allied with digital pulse shaping to improve the confinement of the modulated spectrum toward the Nyquist limit independently of the number of OTDM tributaries used. To investigate the benefits of the proposed approach, we demonstrate the first WDM Nyquist-OTDM signal generation based on the periodic train of sinc pulses and electrical spectrum shaping. Straight line transmission of five 112.5-Gbd Nyquist-OTDM dual-polarization quadrature phase-shift keying (QPSK) channels is demonstrated over a dispersion uncompensated link up to 640 km, with full-field coherent detection at the receiver. It is shown that such a design strategy effectively improves the spectral confinement of the modulated OTDM signal, providing a minimum intercarrier crosstalk penalty of 1.5 dB in baud-rate-spaced Nyquist-WDM systems.

Single-Channel 1.92 Tbit/s, 64 QAM Coherent Nyquist Orthogonal TDM Transmission With a Spectral Efficiency of 10.6 Bit/s/Hz

We have proposed a coherent optical Nyquist pulse and its high-speed optical time division multiplexing (OTDM), in which data signals can be transmitted at a bandwidth as low as the baud rate. By taking advantage of the time-domain orthogonality of coherent Nyquist pulses, Nyquist OTDM signals can be demultiplexed with a high SNR, despite a large intersymbol overlap by adopting Nyquist-LO pulses. With this technique, we successfully transmitted a 1.92 Tbit/s, polarization-division-multiplexed 64 quadrature amplitude modulation Nyquist pulse signal over 225 km within a signal bandwidth of 170 GHz, leading to a potential spectral efficiency of as high as 10.6 bit/s/Hz.

Wavelength Translation of Dual-Polarization Phase-Modulated Nyquist OTDM at Terabit/s

Wavelength translation is one of the key functions to realize spectral defragmentation in an elastic optical add/drop network. Optically translating the wavelength of Terabit/s signals is challenging in the sense that it requires a signal processing scheme to be independent of the symbol rate and modulation format, wideband, and highly efficient simultaneously. In this paper, we present an all-optical wavelength translation for a single-carrier 1.376-Tb/s dual-polarization quadrature phase-shift keying (DP-QPSK) Nyquist optical time-division multiplexing (OTDM) signal by using an all-optical wavelength converter which fulfills all of the basic requirements. A seamless wavelength translation covering almost C-band is achieved with a boost in OSNR thanks to the improved conversion efficiency. The system performance is measured by a polarization-insensitive coherent receiver using linear sampling which implements simultaneously the demultiplexing of the time-domain tributaries, polarization demultiplexing, and carrier-phase recovery for the dual-polarization phase-modulated OTDM signal. The employed receiving scheme, therefore, resolves one of the most shortcomings of the conventional OTDM receiver, which is polarization sensitive due to the use of nonlinear sampling.

Optical Nyquist Filtering for Elastic OTDM Signals: Fundamentals and Demonstrations

Optical Nyquist filtering offers an efficient method to generate ultra-high speed optical time division multiplexing (OTDM) signals with bandwidth equal to the symbol rate without intersymbol interference. Since the Nyquist filtering can be simply implemented in a passive optical device such as a liquid crystal on silicon-based switching element, optical Nyquist filtering is potentially advantageous over digital filtering method in terms of energy efficiency and capacity. Especially in extremely coarse granular optical networks based on high baud rate Nyquist-OTDM signals, it would offer better spectral efficiency and elasticity through efficient add/drop multiplexing with small guard band and flexibility in bandwidth allocation. In this paper, we will discuss important roles of the optical Nyquist filtering as a platform to realize ultra-coarse granular elastic networks based on wavelength division multiplexing of Nyquist OTDM signals. While concepts of basic building blocks, such as elastic multi-Tbit/s transceiver and add/drop multiplexing, are presented, we will introduce proof-of-concept experimental demonstrations placing emphasis on the elasticity and the network aspect operations. We will also argue that spectral defragmentation is a critical network functionality at elastic add/drop nodes to improve the spectral utilization, and the realization of the spectral defragmentation by means of an all-optical wavelength converter is superior to the optical-electrical/electrical-optical approach thanks to the modulation format independent property of the employed optical parametric processes.

Simultaneous two-distributary-channel demultiplexing of an OTDM signal using a bidirectionally operated highly nonlinear fiber after 100km transmission

Abstract A simultaneous two-channel optical time division multiplexing (OTDM) demultiplexing is proposed and experimentally demonstrated by using a bidirectionally operated highly nonlinear fiber (HNLF) followed by a narrow-band offset filter. The performance of the proposed demultiplexer is evaluated in a 40 Gb/s OTDM 100 km transmission system. A maximum power penalty of 2.4 dB is obtained for the worst demultiplexed channel. The proposal might be interesting since it offers a powerful tool for developing ultrafast photonic networks.

Effective optical clock recovery and simultaneous fourfold demultiplexing of OTDM signal using an optoelectonic oscillator

We propose and demonstrate a novel scheme for clock recovery and simultaneous fourfold optical time-division demultiplexing using a dual-parallel Mach-Zehnder modulator based optoelectronic oscillator. 25-GHz prescaled optical clock with a 23% duty cycle and a 22-dB extinction ratio is successfully extracted from both 100-Gb/s on-off keying (OOK) and differential phase-shift keying (DPSK) optical time-division-multiplexing (OTDM) signal. The timing jitters (100 Hz to 10 MHz) are measured to be 195.9 fs and 125.6 fs for the optical clock extracted from the 100-Gb/s OOK and DPSK signal, respectively. Error-free optical time-division demultiplexing is also achieved simultaneously with clock recovery. By adjusting the phase shifter in the OEO loop, all four channels can be selectively demultiplexed. The power penalties at a bit error rate (BER) of 10⁻⁹ for the four demultiplexed channels are measured to be between 0.8 dB and 1.2 dB for the OOK signal and between 0.9 dB and 1.5 dB for the DPSK signal.

No guard-band wavelength translation of Nyquist OTDM-WDM signal for spectral defragmentation in an elastic add–drop node

We demonstrate a seamless spectral defragmentation in an elastic all-optical add-drop node based on wavelength division multiplexing (WDM) channels of Nyquist optical time division multiplexing (OTDM) signal. A 172 Gbaud Nyquist OTDM signal occupying a 215 GHz range is elastically shifted adjacent to its neighboring channel, completely filling a variable spectral gap caused by the dropped channel. The frequency shift is done in a dual-stage polarization-diversity four wave mixing-based converter using polarization-maintaining highly nonlinear fiber. The spectrally defragmented signals are successfully transmitted over a 80 km fiber link with BER<10(-9).

Reconfigurable All-Optical OTDM-to-WDM Conversion Using a Multiwavelength Ultrashort Pulse Source Based on Raman Compression

We propose and demonstrate a reconfigurable all-optical format conversion from optical time-division multiplexing (OTDM) to wavelength division multiplexing (WDM) providing full flexibility in terms of channel spacing and wavelength allocation. The system consists of a Raman amplification-based multiwavelength pulse compressor and a parametric sampling gate. Four conventionally generated WDM 10 GHz pulse trains are compressed at the same time to around 2.5 ps and, then, used for multiwavelength sampling of a 40 Gb/s 3.49 ps OTDM signal. Simultaneous extraction of all four 10 Gb/s tributaries to WDM channels in WDM grid with tunable channel spacing is demonstrated. Controlling time delays among the WDM clock channels makes it flexible to map OTDM tributaries onto any desired wavelength order, which is also called wavelength- and time-selective function. The reconfigurable functions can be realized independently on the coming OTDM signal by simply setting corresponding parameters of the WDM synchronous pulse trains at the terminals. This is also the key for the good overall performance obtained for all demultiplexed channels. Low power penalties below 1.9 and 2.1 dB are achieved with small penalty variations, 0.3 and 0.5 dB, among channels as the OTDM tributaries are converted to WDM grid with spacing of 400 and 200 GHz, respectively.

Single-Carrier 800-Gb/s 32 RZ/QAM Coherent Transmission Over 225 km Employing a Novel RZ-CW Conversion Technique

We report the single-carrier 800-Gb/s coherent transmission of polarization-multiplexed 32 return-to-zero (RZ)/quadrature amplitude modulation signals at 10 Gsymbol/s × 8 optical time-division multiplexing (OTDM). Transmission over 225 km was achieved by adopting an RZ-continuous-wave conversion scheme in a coherent receiver, which made it possible to demodulate and demultiplex ultra high-speed multilevel OTDM signals with a high signal-to-noise ratio.

Investigation of optical clock division and demultiplexing based on a commercial lithium niobate‐intensity modulator

AbstractWe demonstrate and investigate a simple technique to realize multichannel optical clock division by using a commercial lithium niobate‐intensity modulator (LN‐IM). Optical clock division is achieved by controlling the driving and dc bias voltage of the LN‐IM. 20‐ to 10‐GHz optical clock division experiments of both single and double channel are implemented to verify the performance of the proposed optical clock divider. The phase noise of the extracted 10‐GHz optical clock is less than 95 dBc/Hz. The other function of this technique, demultiplexing of optical time division multiplexing (OTDM) signal is also demonstrated experimentally. The 20‐Gb/s PRBS OTDM signal is successfully demultiplexed to two 10 Gb/s with high quality. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:94–99, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26477

Error-free OTDM demultiplexer using the supercontinuum spectrum-slicing induced clock signal

We experimentally demonstrate an 80-Gb/s optical time-division multiplexing (OTDM) demultiplexing system based on the cross-phase modulation (XPM) effect in high-nonlinearity fibers. The message carried by OTDM signal is loaded onto the probe signal by sampling the OTDM packet with a 10-GHz stable and controllable clock pulse train. The clock signal with ~ 11 ps pulse width is achieved by employing supercontinuum spectrum-slicing technique, which is distinct from that based on mode-locked lasers. The demultiplexed signal is obtained by filtering out the XPM-induced spectral sidebands of the probe signal. The OTDM demultiplexer based on this novel technique shows excellent performances and contributes to a bit error rate of as low as 10 −9.

Single Harmonically Driven Electroabsorption Modulator for OTDM Demultiplexing

Optical demultiplexing from 160 Gbit/s to 10 Gbit/s in a single lumped, electrically driven electroabsorption modulator (EAM) is experimentally presented. A quantum-dash Fabry-Pèrot mode-locked-laser is used to sub-harmonically extract a clock signal from the 160 Gbit/s data stream. A drive signal consisting of a series of harmonics of the recovered clock signal enables error-free demultiplexing of all channels in a 160 Gbit/s single-polarization optical time division multiplexing (OTDM) signal. This technique is quasi-analytically compared with the conventional approach of cascading two EAMs when performing 160 Gbit/s to 10 Gbit/s demultiplexing. The analysis reveals that using a single, harmonically driven EAM can result in lower penalties with respect to both degradations of the extinction ratio and width of pulses used in the OTDM signal.

Simultaneous clock recovery for 160 Gbit/s optical time-division multiplexing signal using phase modulation

A phase-comparator-free clock recovery is proposed based on adding the base-rate clock to the optical time-division multiplexing signal through phase modulation. Successful retrieval of 10 GHz base-rate clock and error-free operation of 160 to 40 Gbit/s demultiplexing are demonstrated.