Km Of Standard Single-mode Fiber Research Articles

Demonstration of 108 Gb/s Duo-Binary PAM-8 Transmission and the Probabilistic Modeling of DB-PAM-M BER

We report 108 Gb/s Duo-Binary PAM-8 (DB-PAM-8) transmission using direct-detection aided by Volterra equalizer. The bit-error-rate (BER) performance of the DB-PAM-8 signal has been evaluated through error counting by varying the bit rate, received optical power, and reach. The experimental results suggest that up to 108, 102, and 84 Gb/s signals can be propagated in back-to-back (B2B), and over 10, and 20 km of standard single-mode fiber (SMF), respectively, below the low-density parity-check forward error correction (LDPC-FEC) threshold. Furthermore, we derive a probabilistic model of the BER for multilevel DB-PAM-M signaling that is verified using our measurements. The model of this important performance metric turned out to be accurate enough, especially at low BER values, where its use is more convenient.

Physical Layer Encryption for WDM Optical Communication Systems Using Private Chaotic Phase Scrambling

Protecting the security of optical network is a major worldwide challenge. The conventional schemes for securing communications are based on cryptography at the MAC layer and its higher layers, which are facing a great threat with the development of quantum computers. Optical encryption is a promising way for building security on the physical layer of optical communications. In this work, we propose a novel optical encryption scheme for wavelength division multiplexing (WDM) fiber-optic communication systems, by utilizing the private chaotic phase scrambling. Secure transmission of 50 Gbps signal over 50-km standard single-mode fiber is experimentally and numerically demonstrated. The results show that the WDM signal is efficiently encrypted into a noise-like signal, due to the spectral broadening effect of chaotic phase scrambling. Since the processes of encryption and decryption are totally implemented in the optical domain, the proposed scheme can encrypt the entire network traffic with low latency and high speed. Moreover, the proposed encryption scheme is compatible with the existing WDM optical networks, and only one pair of encryption and decryption devices is sufficient to encrypt all WDM channels, thus the scheme can be easily implemented at low hardware cost.

100-GBd Waveguide Bragg Grating Modulator in Thin-Film Lithium Niobate

An integrated electro-optic modulator based on a waveguide Bragg grating in thin-film lithium niobate is introduced. The compact device with a footprint of 10 × 400 μm2 displays an optical extinction ratio of 53.8 dB at a wavelength of 1555.9 nm in the C-band. With a simple intensity modulation and direct detection scheme, data rates up to 100 Gbit/s are demonstrated with 2-, 4- and 8-level pulse-amplitude modulation formats. The optical filter characteristic of a waveguide Bragg grating modulator suppresses one optical sideband, resulting in inherent single sideband modulation. This enables transmission of a 100 Gbit/s on-off keying signal over 12 km of standard single-mode fiber at 1559.05 nm, without the need for chromatic dispersion compensation.

InAs/InP Quantum Dash Semiconductor Coherent Comb Lasers and their Applications in Optical Networks

We report on the design, growth, and fabrication of InAs/InP quantum dash (QD) gain materials and their use in lasers for optical network applications. A noise performance comparison between QD and quantum well (QW) Fabry–Perot (F-P) lasers has been made. By using the QD gain material we have successfully developed and assembled C-band coherent comb laser (CCL) modules with an electrical fast feedback loop control system to ensure a targeted mode frequency spacing. The frequency spacing was maintained within ±100 ppm and the operation wavelengths locked on the desired ITU grid within 0.01 nm over a period of several months. We also investigated a 25-GHz C-band QD CCL with an external cavity self-injection feedback locking (SIFL) system to reduce the optical linewidth of each individual channel to below 200 kHz in the wavelength range from 1537.55 nm to 1545.14 nm. The RF mode beating signal 3-dB bandwidth was also reduced from 9 kHz to approximately 500 Hz with this SIFL system. These QD CCLs with ultra-low relative intensity noise (RIN), ultra-narrow optical linewidth, and ultra-low timing jitter are excellent laser sources for multi-terabit optical networks. Using a 34.2 GHz QD CCL we demonstrate 10.8 Tbit/s (16QAM 48 × 28 GBaud PDM) coherent data transmission over 100 km of standard single mode fiber (SSMF) and 5.4 Tbit/s (PAM-4 48 × 28 GBaud PDM) aggregate data transmission capacity over 25 km of SSMF with error-free operation.

Using DSP-based effective wireless ethernet accumulation of networks

Mobile fronthaul is an important segment of the network that bridges wireless baseband and remote radio units to support network access to cloud radio. We review recent developments in the use of frequency-division multiplexing to achieve highly bandwidth-efficient, low latency mobile fronthauling. Digital signal processing (DSP) techniques are described for frequency domain windowing, adjacent channel leakage ratio reduction, channel aggregation and deaggregation and synchronous transmission of both the control words (CWs) used for control and management, wireless signal I / Q waveforms. In this experiment, we reveal the transmission of 48 20-MHz LTE signals with a common public radio interface (CPRI) identical data rate of 59 Gb / s, attained a short round-trip DSP latency of < 2 μs, and a small mean error-vector magnitude (EVM) of ∼ 2.5% behind fiber transmission. In a follow-up experiment, together with CPRI-compliant CWs, we further demonstrate the transmission of 32 20-MHz LTE signals, corresponding to a CPRI-equivalent data rate of 39.32 Gb / s, in a single optical wavelength channel which requires an RF bandwidth of only 1.6 GHz. The CWs are recovered without error after transmission over 5 km of standard single-mode fiber, while the LTE signals are recovered with an EVM of<3 per cent. It is also discussed how to apply this technique to future 5 G wireless networks with massive multiple-input multiple outputs. This efficient mobile fronthaul technique may find promising applications for providing ultrabroadband access services in future integrated fiber / wireless access networks.

Employing a Kramers–Kronig receiver with a directly-modulated laser

The Kramers–Kronig (KK) receiver has been used for dispersion compensation in direct detection systems employing external-modulator-based transmitters. In this paper, we investigate the use of the KK receiver for dispersion compensation in a direct detection system based on a directly-modulated laser. We show the validity of the KK receiver by considering a linear contribution to the optical phase arising from a numerical solution of the laser rate equations as a frequency shift of the optical spectrum. The modulated optical signal can then meet the minimum-phase condition required by the KK algorithm. A system model is presented based on the direct modulation of a 10 Gb/s distributed feedback laser with 16-ary quadrature amplitude modulation (16-QAM) subcarrier modulation. The modulated signal is optically filtered, and a 53.5 Gb/s vestigial sideband signal is transmitted over 65 km of standard single-mode fiber. Dispersion compensation is applied after direct detection with a KK receiver. In addition, the impact of the optical filter parameters and receiver sampling rate on system performance are investigated.

Transmission method of improved fiber nonlinearity tolerance for probabilistic amplitude shaping.

For probabilistic amplitude shaping (PAS), we propose a super-symbol transmission method that improves fiber nonlinearity tolerance. A simple fiber nonlinearity low-pass filtering model, as well as its interaction with the spectral dip of signal's intensity waveform, is provided to explain the origin of this nonlinear benefit. With 25-GHz-spaced, 26 × 22.5 GBaud dual-polarized PAS-64 quadrature amplitude modulation (QAM) signals transmitted over 12 spans of 80-km standard single mode fiber (SSMF), the proposed method is found to provide ∼0.15-dB gain over the previous finite-blocklength method with intra-DM pairing, ∼0.26-dB gain over finite-blocklength method with inter-DM pairing, and ∼0.44-dB benefit over the traditional method, all with a feasible blocklength at 200.

Multilevel Manchester (2 bits/symbol) coding with optical equaliser for increasing the transmission distance of high‐speed fibre optic links

This study accessed the performance of the four-level Manchester coding (4-MC) and four-level modified Manchester (4-MM) technique at the bit rates of 20 and 40 Gb/s for the first-order and second-order optical equalisers. The performances were evaluated with respect to bit error rate (BER), power penalty, eye diagrams, and dispersion tolerance. Based on the results by utilising the second-order optical equaliser, at 20 Gb/s, the transmission distance of 4-MC and 4-MM was extended by 25 and 27 km of standard single-mode fibre (SSMF), respectively, while at the bit rate of 40 Gb/s, the 4-MC and 4-MM transmission distance was extended by 6.5 and 7.5 km of SSMF, respectively. Hence, the second-order optical equaliser can adequately compensate for the fibre dispersion and consequently the overall BER performance. Therefore, the proposed system could serve as an alternative to the next-generation high-speed optical transmission links.

Characterization and Direct Modulation of a Multi-Section PIC Suited for Short Reach Optical Communication Systems

A multi-section active photonic integrated circuit (PIC) is characterized in detail to gauge its suitability as a transmitter for short reach applications. The PIC is 1.5 mm long and consists of two lasers integrated in a master-slave configuration, which enables optical injection locking (OIL) of the slave laser. The beneficial impact of the injection is characterized by static and dynamic measurements. The results show a reduction of the optical linewidth from 8 MHz to 2 MHz, a relative intensity noise (RIN) value as low as −154.3 dB/Hz and a 45% improvement of the slave laser modulation bandwidth from 9.5 GHz to 14 GHz. This frequency response enhancement allows the direct modulation of the slave gain section at a data rate of 10.7 Gb/s and an error-free transmission over 25 km of standard single-mode fiber (SSMF). Transmission performance of the injected case shows a 2 dB improvement in the minimum optical power required to achieve a bit error rate of 3.8×10−3 (hard decision forward error correction limit). These results demonstrate that the multi-section PIC can serve as an attractive cost-efficient transmitter in a wide variety of low-cost short-reach data communication applications.

Efficient Time-Bin Encoding for Practical High-Dimensional Quantum Key Distribution

High-dimensional quantum key distribution (QKD) allows achievement of information-theoretic secure communications, providing high key-generation rates, which cannot, in principle, be obtained by QKD protocols with binary encoding. Nonetheless, the amount of experimental resources needed increases as the quantum states to be detected belong to a larger Hilbert space, thus raising the costs of practical high-dimensional systems. Here, we present an alternative scheme for fiber-based four-dimensional QKD, with time and phase encoding and one-decoy state technique. Quantum state transmission is tested over different channel lengths up to 145 km of standard single-mode fiber, evaluating the enhancement of the secret key rate in comparison to the three-state two-dimensional BB84 protocol, which is tested with the same experimental setup. Our scheme allows measurement of the four-dimensional states with a simplified and compact receiver, where only two single-photon detectors are necessary, thus making it a cost-effective solution for practical and fiber-based QKD.

Wideband QAM-over-SMF/turbulent FSO downlinks in a PON architecture for ubiquitous connectivity

We propose and investigate for the first time a seamless millimeter-wave (mmW) radio-over-fiber (RoF) and radio-over-free-space optics (FSO)-based downlink for use in a passive optical network architecture using 4-, 16- and 64-quadrature amplitude modulation (QAM) for broadband wireless access (BWA) networks. The proposed system is implemented in both experiment and simulation to realize continuous and ubiquitous coverage in urban and rural areas. We outline, a proof-of-concept demonstration of 4-, 16- and 64-QAMs at data rates of 34, 67 and 100 Mb/s, respectively transmitted over a 15 km standard single-mode fiber (SMF), which is then optically up-converted to 25 GHz for transmission over a 10 km of SMF and a 2 m of FSO channel under a non-uniform turbulent condition. We show the measured error vector magnitude (EVM) values of 13, 9.2 and 7.3% for 4-, 16- and 64-QAM, respectively, which are below the corresponding standard EVM requirements and therefore confirm the practicality of the proposed hybrid system. Depending on the data rates, each modulation can be adaptively configured. We report a simulation of a 10 Gb/s 4- and 64-QAM hybrid RoF-FSO downlink under an extended non-uniform turbulence regime to verify the feasibility of the proposed scheme for use in practical applications. By implementing the decision-directed carrier phase recovery and linear electrical equalization, EVMs can be efficiently reduced below the required limits. We further evaluate the proposed system performance in terms of the bit error rates, constellation diagrams, received optical and mmW powers. Using state-of-the-art K-band power amplifiers and conical horn antennas, the maximum wireless transmission distance is estimated to be about 135 m for use in the last-mile BWA networks.

Nonlinearity Tolerant LUT-Based Probabilistic Shaping for Extended-Reach Single-Span Links

We propose Huffman-coded sphere shaping (HCSS) as a method for probabilistic constellation shaping which provides improved tolerance to fiber nonlinearities in single-span links. An implementation of this algorithm based on look-up-tables (LUTs) allows for low-complexity, multiplier-free shaping. The advantage of short-length shaping for mitigating fiber nonlinear impairments is experimentally demonstrated for a system employing dual–polarization 64–ary quadrature amplitude modulation (DP-64QAM) at 56 GBd and operating over 210 km of standard single-mode fiber (SSMF). A gain in achievable information rate (AIR) of 0.4 bits/4D-symbol compared with uniform signaling is measured, corresponding to a 100% improvement in shaping gain compared with ideal Maxwell–Boltzmann (MB) shaping. The combinatorial mapping and demapping algorithms can be implemented with integer addition and comparison operations only, utilizing an LUT with 100 kbit size.

Employing signal-to-noise-and-distortion ratio for power loading of a directly modulated laser in an FDM system

For the direct modulation of a laser using frequency division multiplexed (FDM) channels, an iterative procedure is presented for power loading based on the signal-to-noise-and-distortion ratio. The power of nonlinear distortion arising from the modulation dynamics of the laser and dispersive transmission combined with direct detection is estimated using a two-tone test with the power being determined within the receiver bandwidth for each FDM channel. This allows the signal distortion to be isolated and provides a generic result that avoids having to specify the modulation format and execute system-level simulations. The noise arising from the receiver front-end is described by an equivalent thermal noise. The power loading procedure is tested numerically with 16 FDM channels using 8-ary phase shift keying (8-PSK), 16-ary quadrature amplitude modulation (16-QAM), or 32-QAM. The resulting FDM signal is transmitted over 30 km of standard single-mode fiber followed by direct detection. In the case of using 16-QAM, 60.8 Gb/s transmission is achieved, and despite the frequency-selective power fading, all channels perform under the target bit error ratio of 3.8×10−3. Moreover, the robustness of the power loading profile is evaluated by changing the transmission distance while using a profile optimized for a specific distance.

M-QAM signal transmission at the photonically generated K-band over thermal-induced turbulent FSO links with different turbulence distributions

We present a theoretical and experimental study on the impact of different thermal-induced free-space turbulence distributions on the M-quadrature amplitude modulation (M-QAM) signal transmission in radio frequency K-band over hybrid optical links of standard single mode fiber (SSMF) and free-space optics (FSO). Frequency multiplication using an external intensity modulator biased at the null transmission point has been employed to photonically generate radio signals at a frequency of 25 GHz , included for the frequency bands for fifth-generation (5G) mobile networks. Moreover, extensive simulations have been performed for 10Gb/s with 4-, 16-, and 64-QAM over 5 km of SSMF and 500 m long FSO channels under scenarios with different turbulence levels and distributions. Proof-of-concept experiments have been conducted for 20 MHz with 4- and 64-QAM over 5 km of SSMF and 2 m long FSO channels under turbulence conditions. Both theoretical and experimental systems have been analyzed in terms of error vector magnitude (EVM) performance showing feasible transmission over the hybrid links in the received optical power range. Non-uniform turbulence distributions are shown to have a different impact on M-QAM modulation formats, i.e., turbulence distributions with higher strength in the middle of the FSO link reveal a 1.9 dB penalty when using 64-QAM signals compared to a 1.3 dB penalty using 4-QAM signals, whereas higher penalties have been measured when 4-QAM format is transmitted over turbulence distributions with larger magnitude in the second half of the FSO link. The results have been validated by theoretical predictions and lead to practical consequences on future networks' deployment.

DCI Field Trial Demonstrating 1.3-Tb/s Single-Channel and 50.8-Tb/s WDM Transmission Capacity

We demonstrate, to the best of our knowledge, the highest reported single- and multi-channel capacities over a field-deployed data center interconnection (DCI) system operating over 93 km of standard single-mode fiber using only Erbium-doped fiber amplification. Using off-the-shelf hardware components, and in particular using only a single digital-to-analog converter per complex signal dimension, we achieve a net bit rate of 1.3 Tb/s per wavelength (single-channel operation) and more than 1.25 Tb/s per channel in a 40-channel wavelength-division multiplexed (WDM) system with a spectral efficiency of 11.29 bits/s/Hz. We achieve a total WDM capacity of 50.8 Tb/s across the C-band.

Experimental Demonstration of Dual O+C-Band WDM Transmission Over 50-km SSMF With Direct Detection

In this article, we experimentally demonstrate dual O+C-band direct-detection wavelength division multiplexed (WDM) transmission over 50 km of standard single-mode fibre, wherein 4 × 40-Gb/s Nyquist on-off keying (OOK) modulation is used in the O-band whilst 4 × 40-Gb/s single-sideband (SSB) subcarrier modulation (SCM) with Kramers–Kronig (KK) detection is used in the C-band. A bismuth-doped fibre amplifier (BDFA) is used as the pre-amplifier in the O-band to extend the reach of the direct-detection system. It is demonstrated that the O-band exhibits superior receiver sensitivity relative to the C-band and its transmission performance is limited by the chromatic dispersion (CD), especially when longer reaches and wavelengths in the longer edge of the band are explored. In contrast, due to the adoption of the SSB format, the C-band shows enhanced tolerance to CD effects. In this case, some receiver sensitivity is sacrificed to accommodate the adoption of the KK detection which is used to eliminate the signal-to-signal beating interference of the SSB-SCM format. The inferior receiver sensitivity in the C-band is caused by (1) the higher CSPR requirement of the KK detection and (2) the restriction of small-signal modulation for the dual-drive Mach–Zehnder modulator to generate optical SSB signals. The results indicate the feasibility to further extend the reach of O-band transmission systems, which enables the possibility to realise dual O+C-band WDM transmission in intermediate- to long-haul optical networks.

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.

Digital Resolution Enhancer Employing Clipping for High-Speed Optical Transmission

Driven by the exponentially increasing demand for Internet traffic, single channel capacity in optical transmission systems continues to grow. Therefore, optical transmitter impairments such as low analog bandwidth, nonlinearities and digital-to-analog conversion with low resolution place a considerable burden on the system design. Methods aimed at compensating for the low transmitter bandwidth and transmitter nonlinearities have been introduced and recently, a digital resolution enhancer (DRE) which reduces the quantization noise added by a digital-to-analog converter (DAC) has been proposed. In this work, the interplay between DRE and clipping is introduced and studied. Experimental validation is performed by transmitting single channel 600 Gb/s 64-quadrature amplitude modulation (QAM) and wavelength-division multiplexing 460 Gb/s 128-QAM over 75 km of standard single-mode fiber. The combination of DRE and clipping allows the reduction of the DAC's physical number of bits to 3, while still maintaining performance above the forward error correction (FEC) limit. Low resolution DACs will play an important role in the implementation of low-power transmitters generating high-cardinality modulation formats.

Chromatic dispersion mitigation using a SEFDM-based diversity technique for IM/DD long reach optical links.

Spectral efficient frequency division multiplexing (SEFDM) can offer a higher spectral efficiency (SE) than orthogonal frequency division multiplexing (OFDM). In this work, we propose a diversity technique based on SEFDM for beyond 100-Gb/s optical intensity modulation and direct detection (IM/DD) long reach (LR) applications. We mathematically demonstrate that the self-created inter-carrier interference of SEFDM signals can be reused to achieve a diversity gain on each sub-carrier and, in turn, improve the tolerance to power fading induced by chromatic dispersion (CD) in IM/DD LR links. Based on the proposed diversity technique, we further demonstrated a 112-Gb/s SEFDM transmission over 80-km standard single-mode fiber, using only 28-GHz bandwidth and modulation format of up to 16-QAM. Experimental results show that SEFDM with the proposed diversity technique performs robust against CD effects and outperforms the conventional OFDM with adaptive bit and power loading of the same bandwidth and data rate, which validates the superiority of the proposed SEFDM in optical IM/DD LR transmissions.

100-Gb/s DMT Transmission for Short-Reach Optical Interconnect With DAC Clock Leakage Compensation

Digital-to-analog converters (DAC) with a high sample rate and large bandwidth are preferred to achieve high data-rate signal generation in an optical discrete multi-tone (DMT) system. Clock leakage is one of the imperfections in high-speed DACs. One or more data subcarriers of the generated DMT signal may be interfered by the additional clock tone, which may cause error floor in bit error rate (BER) performance. In this work, the pre-equalized 16/32QAM-DMT signals with the net data rate up to 121 Gbit/s are generated by an 80 GSa/s DAC with a clock leakage at 20 GHz. Two simple DAC clock leakage compensation (CLC) methods are employed and investigated in the DMT receiver. The results exhibit the two CLC methods have a similar performance. After 2 km of standard single-mode fiber (SSMF) transmission, by using the two CLC methods, the BER performance can be improved by up to about an order of magnitude and below 3.8e-3 for 16QAM-DMT. The receiver sensitivity in terms of the received optical power can be improved by 2-dB in 32QAM case at the BER of 2e-2. Furthermore, the BER performance with two CLC methods has good stability over the measurement period.