Optical Time Division Multiplexing Research Articles

Multiplexing in photonics as a resource for optical ternary content-addressable memory functionality

Abstract In this paper, we combine a Content-Addressable Memory (CAM) encoding scheme previously proposed for analog electronic CAMs (E-CAMs) with optical multiplexing techniques to create two new photonic CAM architectures—wavelength-division multiplexing (WDM) optical ternary CAM (O-TCAM) and time-division multiplexing (TDM) O-TCAM. As an example, we show how these two O-TCAM schemes can be implemented by performing minor modifications in microring-based silicon photonic (SiPh) circuits originally optimized for exascale interconnects. Here, our SiPh O-TCAM designs include not only the actual search engine, but also the transmitter circuits. For the first time, we experimentally demonstrate O-TCAM functionality in SiPh up to ∼ 4 Gbps ${\sim} 4\,\,\text{Gbps}$ and we prove in simulation feasibility for speeds up to 10 Gbps, 10 times faster than typical E-TCAMs at the expense of higher energy consumption per symbol of our O-TCAM Search Engine circuits than the corresponding E-TCAMs. Finally, we identify which hardware and architecture modifications are required to improve the O-CAM’s energy efficiency towards the level of E-CAMs.

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.

Performance improvements of a VLC system, in a V2X context, using a different multiplexing technique

This paper scrutinizes visible light communication (VLC) employing hybrid optical time division multiplexing (OTDM) wavelength division multiplexing (WDM). It also demonstrates the simulation of VLC using the direct sequence optical code division multiple access (DS-OCDMA) system developed by OptiSystem-Matlab Co-simulation. The profits of WDM and OTDM capacities have been combined using two levels of freedom to build up of the system. The performances are demonstrated in the simulated findings of the bit error rate (BER) in relationship with the data rate and the number of users. First, the simulated results display the usefulness of the system. Then, the number of users has been significantly enhanced. Besides, the OCDMA functions by appointing a specific code to each user and could provide asynchronous and simultaneous access to numerous users. One drawback is multiple access interference (MAI). The use of very long code sequences is crucial in order to lessen this effect. Hence, very large bandwidths are essential. This study aims at demonstrating, through system simulation, the practicability of an all-optical conventional correlation receiver dedicated to a DS-OCDMA access network link. Again, OptiSystem has been used to realize our blocks.

Optical cascaded reservoir computing for realization of dual-channel high-speed OTDM chaotic secure communication via four optically pumped VCSEL.

In this work, we propose a chaotic secure communication system with optical time division multiplexing (OTDM), using two cascaded reservoir computing systems based on multi beams of chaotic polarization components emitted by four optically pumped VCSELs. Here, each level of reservoir layer includes four parallel reservoirs, and each parallel reservoir contains two sub-reservoirs. When the reservoirs in the first-level reservoir layer are well trained and the training errors are far less than 0.1, each group of chaotic masking signals can be effectively separated. When the reservoirs in the second reservoir layer are effectively trained and the training errors are far less than 0.1, the output for each reservoir can be well synchronized with the corresponding original delay chaotic carrier-wave. Here, the synchronization quality between them can be characterized by the correlation coefficients of more than 0.97 in different parameter spaces of the system. Under these high-quality synchronization conditions, we further discuss the performances of dual-channel OTDM with a rate of 4×60 Gb/s. By observing the eye diagram, bit error rate and time-waveform of each decoded message in detail, we find that there is a large eye-openings in the eye diagrams, low bit error rate and higher quality time-waveform for each decoded message. Except that the bit error rate of one decoded message is lower than 7 × 10-3 in different parameter spaces, and those of the other decoded messages are close to 0, indicating that high-quality data transmissions are expected to be realized in the system. The research results show that the multi-cascaded reservoir computing systems based on multiple optically pumped VCSELs provide an effective method for the realization of multi-channel OTDM chaotic secure communications with high-speed.

300-Gb/s/λ IM/DD Transmission Using Integrated SiP OTDM Transmitter

We fabricate an integrated silicon-photonic (SiP) 2-channel optical-time-division-multiplexing (OTDM) transmitter, which is operable with a sinusoidally modulated input light source. This SiP transmitter consists of two Mach-Zehnder modulators (MZMs), two 1×2 multimode interferometer (MMI) couplers, a time-delay line (TDL), and a thermo-optic phase shifter (TOPS). The 3-dB bandwidth of these MZMs is measured to be as large as 60 GHz. However, due to the limited bandwidths of the various components used in our experiment, the overall bandwidth of each OTDM channel is reduced to ~35 GHz. Nevertheless, by using this SiP transmitter, we could demonstrate the transmission of a 300-Gb/s OTDM pulse-amplitude modulation 8-level (PAM8) signal over 500 m of the standard single-mode fiber (SSMF). We also confirm that the fabricated SiP OTDM transmitter can be utilized even with the fabrication fluctuations of the TDL.

Digital inter-symbol interference mitigation method for Nyquist OTDM signal detection with coherent sampling and KNN classifier

We propose and experimentally demonstrate a digital method to mitigate inter-symbol interference (ISI) in demultiplexing Nyquist optical time-division multiplexing (N-OTDM) signal using a simple but powerfully serviceable k-nearest neighbors (KNN) classifier. The proposed method is evaluated under four types of sampling pulse-width corresponding to various extents of ISI distortion in 160 Gbaud down to 40 Gbaud demultiplexing experiments, and the mitigation performance is found to be better along with the increasement of sampling pulse-width. Compared with the traditional decision-directed least mean square (DD-LMS) equalization method, a 3.9 dB required-OSNR (at a BER of 10−4) improvement is observed under the pulse-width of 5.3 ps. Demultiplexing performance is investigated under the pulse-widths of 3.2 ps and 3.9 ps as well, in which the improvement of required-OSNR are found to be 1.33 dB and 2.95 dB, respectively. The performance improvement clearly indicates the strong ISI mitigation ability of the KNN classifier for N-OTDM signal demultiplexing. In addition, experimental results also confirm that the proposed digital ISI mitigation method could release the pulse-width requirement of the N-OTDM demultiplexing, which paves the way for future practical application of N-OTDM technique.

A Review on Nonlinearity in Semiconductor optical Amplifer as Application in all Optical Signal Processing

The optical signal processing is the future of communication network. High speed all-optical packet routing is one of the essential applications of all-optical networks which is only possible with the development of all-optical logic technology. The development of optical logic elements and circuits are the steps in the growth of this technology and higher speed up to Tbit/sec can be achieved. The optical carrier frequency range 1013 to 1016 Hz provides enormous potential bandwidth with superior information carrying capacity over a long transmission distance. The need of higher capacity is continuing to encourage research in wavelength division multiplexing (WDM) and optical time division multiplexing (OTDM) based transmission systems, which need optical demultiplexing and wavelength conversion technology. Therefore, for high-speed optical networks, it is required to develop the all-optical gates to avoid power consumption in opto-electronics conversion. All optical logic gates perform computing operations, storage and transmission of data using light also known as optical computing. Optical technology promises massive upgrades in the efficiency and speed of computers, aswell as significant shrinkage in their size and cost. The non-linear optical device to be used i.e. Semiconductor Optical Amplifier (SOA) has proved to be the promising for all optical functions like wavelength conversion, logic functions, signal representation in all optical domain. Its compact size, high gain, fast response, strong refractive index variation, easy to manufacture and integration, and power efficiency makes it most optimum device for optical signal processing. In this paper, the application of SOA in optical processing is thoroughly reviewed and orient towards the latest application in neural networks.

Design and analysis of a compact micro-ring resonator signal emitter to reduce the uniformity-induced phase distortion and crosstalk in orbital angular momentum (OAM) division multiplexing.

To improve the transmission capacity of an optical system, different multiplexing schemes have been proposed, such as optical time division multiplexing (OTDM), wavelength division multiplexing (WDM), polarization division multiplexing (PolDM), spatial division multiplexing (SDM), etc. One kind of SDM technique to boost the capacity is through modifying the spatial phase structure of an optical beam, which is known as the orbital angular momentum (OAM) division multiplexing. Moreover, the OAM signal emitter can be produced by using mature and high-yield silicon photonic (SiPh) technology, without the need of using bulky optical components or expensive spatial light modulator (SLM). The SiPh-based micro-ring resonator is one of the promising OAM signal emitter candidates, since it is simple, compact and easy to fabricate. However, the device performance is highly subjected to the structural design, and the uniformity-induced phase distortion will significantly degrade the purities of OAM beams; hence, introducing severe OAM signal crosstalk during the OAM division multiplexing. In this work, a compact SiPh-based micro-ring resonator type OAM signal emitter with detailed design parameters is presented and the output signal uniformity issue is comprehensively investigated. Two kinds of the structural optimization are performed by adjusting the angular grating width as well as the grating height. The results indicate that a significant improvement in output OAM beam uniformity can be achieved, with the attenuation factor being improved over 88% at the price of acceptable 4 ∼ 5% coupling efficiency reduction. The variations of the transmission and the uniformity induced by the fabrication error are also analyzed.

Towards High-Resolution Imaging With Photonics-Based Time Division Multiplexing MIMO Radar

A photonics-based time division multiplexing (TDM) multiple-input and multiple-output (MIMO) radar is proposed to realize high-resolution imaging. In this MIMO radar, multiple transmitters generate time-domain orthogonal linearly frequency modulated (LFM) signals based on microwave photonic frequency octupling and optical TDM technique, and multiple receivers implement dechirping of the multi-channel radar echoes based on microwave photonic frequency mixing. The microwave photonic techniques make the system capable to generate and process broadband radar signals, and the TDM yields a high spectral efficiency, which allows a large bandwidth in each transmitter and receiver. Meanwhile, the TDM-MIMO ensures complete orthogonality between different channels and enables good flexibility in arranging more transmit and receive channels. Therefore, the proposed photonics-based TDM-MIMO radar can achieve both high range resolution and high angular resolution. In the experiment, a photonics-based 4 × 8 TDM-MIMO radar is established which has an operation bandwidth of 8 GHz in each transmit channel. The range resolution and angular resolution are estimated to be 1.9 cm and 1.1°, respectively. A broadband back projection (BP) imaging algorithm with both frequency and time-delay dependent phase compensation is proposed, based on which high resolution imaging of single and complex targets is achieved.

300-Gb/s Transmission Using OTDM System Implemented With Sinusoidally Modulated Input Light Source

The recently proposed 2-channel optical-time-division-multiplexing (OTDM) technique utilizing a sinusoidally modulated input light source provides an attractive solution to double the per-wavelength data rate of the short-reach intensity-modulation/ direct-detection system. However, to maximize the transmission speed of such an OTDM system, it is necessary to generate the sinusoidal input light operating at the frequency much faster than the modulator’s bandwidth. As a result, the extinction ratio (ER) of the generated sinusoidal input light can be significantly reduced, which, in turn, causes a large crosstalk in this OTDM system. To solve this problem, we propose to enhance the ER of the sinusoidal input light simply by decreasing the bias voltage applied to the Mach-Zehnder modulator (MZM) used for its generation (from the quadrature point toward the null point). We estimate that, by using this technique, it is possible to increase the ER from 3.1 dB to 14.9 dB when the required sinusoidal frequency is three times faster than the MZM’s bandwidth. To verify the effectiveness of this ER enhancement technique, we realize the 2-channel OTDM system with a sinusoidally modulated input light source by using the MZMs having a 3-dB bandwidth of 17.2 GHz and demonstrate the transmission of the 300-Gb/s PAM8 signal. Thus, the ratio between the achieved data rate and the 3-dB bandwidth of the MZM is as high as ~17.4.

Performance analysis of an optical TDM transmission link considering fiber dispersion and demultiplexer crosstalk

This paper outlines the performance evaluation of high speed optical time division multiplexed (OTDM) system considering the effect of fiber dispersion and the impairments of nonlinear optical loop mirror (NOLM) used as a demultiplexer. The system performance gets deteriorated by the channel crosstalk and the relative intensity noise (RIN) of the NOLM. Our investigation demonstrates that the system is penalized in terms of power penalty significantly when the intrinsic crosstalk becomes higher than −25 dB. The power coupling ratio must be within the range of 0.47–0.53 in order to maintain the intrinsic crosstalk within −25 dB level. Our analysis also indicates that the system suffers more power penalty with the increasing number of demultiplexing channel and the amount of pulse walk-off between signal and control pulse. Apart from channel crosstalk the system performance is also investigated taking into account the effect of intensity fluctuation of the demultiplexed signal which is called RIN of the NOLM. The system suffers from significant power penalty for the value of such noise higher than 10−5. It is also reported that the power penalty is significant when the chromatic dispersion index becomes higher than 0.30.

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.

InAs/GaAs quantum dot single-section mode-locked lasers on Si (001) with optical self-injection feedback.

Silicon based InAs quantum dot mode locked lasers (QD-MLLs) are promising to be integrated with silicon photonic integrated circuits (PICs) for optical time division multiplexing (OTDM), wavelength division multiplexing (WDM) and optical clocks. Single section QD-MLL can provide high-frequency optical pulses with low power consumption and low-cost production possibilities. However, the linewidths of the QD-MLLs are larger than quantum well lasers, which generally introduce additional phase noise during optical transmission. Here, we demonstrated a single section MLL monolithically grown on Si (001) substrate with a repetition rate of 23.5 GHz. The 3-dB Radio Frequency (RF) linewidth of the QD-MLL was stabilized at optimized injection current under free running mode. By introducing self-injection feedback locking at a feedback strength of -24dB, the RF linewidth of MLL was significantly narrowed by two orders of magnitude from 900kHz to 8kHz.

A Research on Key Technologies in All-Optical Label Switching Networks

With the rapid development of Internet technology, the capacity of communication system is increasing rapidly and traditional switching technology based on optical-electrical-optical (O/E/O) conversion cannot satisfy requirements for information exchange. Construction of a high-speed, large-capacity, high-bandwidth all-optical switching network will be the future direction of communication systems. In this paper, the key technologies in all-optical label switching networks are investigated. First, five kinds of optical label technologies are studied. The principles and implementation methods of them are analyzed and compared. Then, optical switching fabrics based on different kinds of optical switches are discussed. Next, a 160Gb/s Optical Time Division Multiplexing (OTDM) signal 1 × 8 packet switching experiment is introduced. Finally, the research work is summarized, and the design of core nodes in optical packet switching network is prospected.

Optical TDM technology for N+1 redundant systems in high resolution optical earth observation satellites

Optical TDM technology for N+1 redundant systems in high resolution optical earth observation satellites

Proposal for optical TDM system for digital payload in next generation high throughput satellites

The digital payload for next generation high throughput communication satellites is expected to reduce the operators’ CAPEX and OPEX. Photonic technologies, which have a great affinity with digital signals, have been studied in order to increase the data throughput and reduce the mass of the satellites. Here, we propose an optical time division multiplexing system for routing the signals in the digital channelizer and have conducted a feasibility study on the transmission efficiency. This system has the potential to reduce the cost, mass, and power consumption, and to improve reliability by cleaning up the switches used for routing the signals within the satellites.

Receiving sensitivity improvement by wide-spectrum OAM with OTDM and polarization multiplexing in weak atmospheric turbulence.

In this paper, a wide-spectrum orbital angular momentum (OAM) system with a polarization and optical time division multiplexing (OTDM) free-space transmission system is experimentally demonstrated. To enhance the system transmission performance in atmospheric turbulent channel, a wide-spectrum laser and an OAM beam are used. The wide-spectrum laser can be generated by utilizing pumped laser to pump nonlinear fiber, and OAM can be generated with a special light modulator. Furthermore, OTDM and polarization multiplexing methods are used to enhance the communication rate from 4 Gbit/s to 32 Gbit/s. With the use of the wide-spectrum laser and the OAM beam, the receiving scintillation index (SI) can be reduced, and detection sensitivity can be improved. It is the first time a wide-spectrum OAM communication system performance has been studied. It is shown that under weak atmospheric turbulence condition, the SI can be reduced by 38% and the receiving sensitivity can be improved by 3.18 dB via wide-spectrum OAM beams.

Performance Analysis of High Speed Bit-Interleaving Time-Division Multiplexing Passive Optical Networks (TDM-PONs)

Time-division multiplexing passive optical networks (TDM-PONs) considered as a good solution for a high bit rate and flexible bandwidth system. In this paper, the simulation of a new bit-interleaving TDM transmitter has carried out. The proposed scheme of downstream TDM-PON based on single Mach–Zehnder modulators (MZM) and single laser diode to carry an electrical multiplexed data providing cost effective, high-transmitted power and easy implementation system. The TDM-PON technique has seen widespread since the beginning of this century, especially with FTTH where flexible bandwidth and high bit rate are required. Hence, the simulation of 10, 25, 40, and 50 Gbps TDM-PON have been presented in three scenarios based on two downstream transmitter schemes of FTTH. Those scenarios have carried out the standard single-mode fiber (SSMF) and the free-space optic (FSO) as transmission media, and the single mode dispersion compensating fiber (DCF) and Fiber Bragg Gratings (FBG) as dispersion compensators. The results show that the electrical multiplexed scheme of TDM transmitter provides better performance with the comparison of the traditional optical TDM transmitter in different scenarios with different bit rates.

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.

FWM Mitigation in DWDM Optical Networks

In this paper, three optical communication systems have been proposed to mitigate Four Wave Mixing (FWM). Three techniques are used, namely; low input power with high gain amplifier, a collective Optical Time Division Multiplexing (OTDM) and Dense Wavelength Division Multiplexing (DWDM) system, and the use of alternative circular polarization. The first technique involves reduction in input power to -20 dBm and then amplifying it 40 dB before demultiplexing. The second technique divides the input signal into four time slots and then combine them with a power combiner. In the third technique, the polarization of light coming from input channels is changed after modulation into right and left handed circular polarization. Exhaustive sets of simulations are performaed using Optisys. The performance analysis includes Q-factor, Optical Signal to Noise Ratio (OSNR), received optical power, minmum Bit Error Rate (BER) and eye diagram.