dB Power Penalty Research Articles

Multiterabit Transmission Over OM2 Multimode Fiber With Wavelength and Mode Group Multiplexing and Direct Detection

In this paper, 5 Tb/s bidirectional transmission (2.5 Tb/s in each direction) over 2.2 km of OM2 fiber is demonstrated using selective excitation of four mode groups, wavelength division multiplexing, and direct detection. Twenty wavelengths per mode group are used, each wavelength is modulated using discrete multitone scheme with 1024 subcarriers with external lithium niobate Mach–Zehnder modulator (MZM). A minimum bit rate of 68.8 Gb/s per channel is obtained for a bit error rate of 3.8 × 10−2 using a 65 Gs/s digital to analog converter (DAC). Same bit rate is achieved over 4.4 km using 88 Gs/s DAC. Electroabsorption modulated laser (EML) with a 3 dB bandwidth of 18 GHz is also used in the same mode group division transmission. 1 dB power penalty is measured with the EML compared to MZM. A minimum of channel bit rate of 68.5 Gb/s over 2.2 km of OM2 fiber with the EML and 88 Gs/s DAC is reported. Therefore, we demonstrate the transmission of the same throughput over multimode fiber using a low-cost EML instead of an external MZM.

Mode selecting switch using multimode interference for on-chip optical interconnects.

A novel mode selecting switch (MSS) is experimentally demonstrated for on-chip mode-division multiplexing (MDM) optical interconnects. The MSS consists of a Mach-Zehnder interferometer with tapered multi-mode interference couplers and TiN thermo-optic phase shifters for conversion and switching between the optical data encoded on the fundamental and first-order quasi-transverse electric (TE) modes. The C-band MSS exhibits a >25 dB switching extinction ratio and < -12 dB crosstalk. We validate the dynamic switching with a 25.8 kHz gating signal measuring switching times for both TE0 and TE1 modes of <10.9 μs. All channels exhibit less than 1.7 dB power penalty at a 10-12 bit error rate, while switching the non-return-to-zero PRBS-31 data signals at 10 Gb/s.

Simplified demultiplexing scheme for two PDM-IM/DD systems utilizing a single Stokes analyzer over 25-km SMF

We propose a four-linear state of polarization multiplexed intensity modulation and direct detection (IM/DD) scheme based on two orthogonal polarization division multiplexing (PDM) on-off keying systems. We also experimentally demonstrate a simple demultiplexing algorithm for this scheme by utilizing only a single Stokes analyzer. At the rate of 4×10 Gbit/s, the experimental results show that the power penalty of the proposed scheme is about 1.5dB, compared to the single PDM-IM/DD for back-to-back (B2B) transmission. Compared to B2B, just about 1.7dB power penalty is required after 25km Corning LEAF optical fiber transmission. Meanwhile, the performance of the polarization tracking is evaluated, and the results show that the BER fluctuation is less than 0.5dB with a polarization scrambling rate up to 708.75deg/s.

Single Channel 50 Gbit/s Transmission Over 40 km SSMF Without Optical Amplification and In-Line Dispersion Compensation Using a Single-End PD-Based PDM-SSB-DMT System

We experimentally demonstrate a single-end photodetector (PD) based polarization-division-multiplexing (PDM) system, in which the single-sideband orthogonal frequency-division multiplexing (SSB-OFDM) and discrete multitone (DMT) signals are modulated on two orthogonal polarizations respectively. Based on the frequency interleave design for dual-polarization, the spectral efficiency and chromatic dispersion tolerance can be optimized. We derive the theoretical model for the PDM-SSB-DMT system and detail the digital signal processing for demultiplexing at the receiver. We also successfully transmit a 50 Gb/s PDM-SSB-DMT signal with 14 GHz bandwidth over 40 km of uncompensated and unamplified standard single mode fiber (SSMF) with only 1.1 dB power penalty for a bit error rate of 3.8 × 10–3 relative to back-to-back transmissions.

Design of Low-Power DSP-Free Coherent Receivers for Data Center Links

Coherent detection offers high spectral efficiency and receiver sensitivity, but digital signal processing (DSP)-based coherent receivers may be prohibitively power hungry for data centers even when optimized for short-reach applications, where fiber propagation impairments are less severe. We propose and evaluate low-power DSP-free homodyne coherent receiver architectures for dual-polarization quadrature phase shift keying (DP-QPSK) for inter- and intradata center links. We propose a novel optical polarization demultiplexing technique, for DP-QPSK and higher-order modulation formats, with three cascaded phase shifters driven by marker tone detection circuitry. We consider carrier recovery based on either optical or electrical phase-locked loops (PLLs). We propose a novel multiplier-free phase detector based on XOR gates, which exhibits less than 0.5 dB power penalty relative to a conventional Costas loop phase detector. We also study the relative performance of homodyne DP-differential QPSK, for which carrier phase recovery is unnecessary. Our proposed DSP-free architectures exhibit ∼1 dB power penalty at small chromatic dispersion compared to their DSP-based counterparts. We estimate conservatively that the high-speed analog electronics of an electrical PLL-based coherent receiver consume nearly 4 W for 200 Gbit/s DP-QPSK, assuming a 90-nm complementary metal-oxide semiconductor process.

Multi-bit wavelength coding phase-shift-keying optical steganography based on amplified spontaneous emission noise

In this paper, a multi-bit wavelength coding phase-shift-keying (PSK) optical steganography method is proposed based on amplified spontaneous emission noise and wavelength selection switch. In this scheme, the assignment codes and the delay length differences provide a large two-dimensional key space. A 2-bit wavelength coding PSK system is simulated to show the efficiency of our proposed method. The simulated results demonstrate that the stealth signal after encoded and modulated is well-hidden in both time and spectral domains, under the public channel and noise existing in the system. Besides, even the principle of this scheme and the existence of stealth channel are known to the eavesdropper, the probability of recovering the stealth data is less than 0.02 if the key is unknown. Thus it can protect the security of stealth channel more effectively. Furthermore, the stealth channel will results in 0.48 dB power penalty to the public channel at 1×10−9 bit error rate, and the public channel will have no influence on the receiving of the stealth channel.

Adaptation of transmitting signals over joint aged optical fiber and free space optical network under harsh environments

Over the last two decades, a large amount of optical fiber (OF) cables has been deployed as part of the global communication networks. Both the aging of OFs as well as the need to increase transmission data rates, particularly in the backbone, have become hot topics. We present the study of the aged OF deployment in various optical networks including free space optics (FSO) link as a part of modern optical communication networks. Here, we show extended results obtained using a dedicated OF testbed focusing on the long-term monitoring of polarization mode dispersion (PMD) because of its time-varying nature. The adaptation of polarization multiplexed radio over fiber (RoF) and radio over FSO (RoFSO) systems as well as 10Gbps on-off-keying (OOK) non-return-to-zero (NRZ) intensity modulation with the direct detection system, which is common cost-effective transmission system in passive networks, are demonstrated. Moreover, simulation of 100 and 200Gbps return-to-zero (RZ) differential quadrature phase shift keying (DQPSK) with direct detection is outlined to verify the impact of aged OF network connected with FSO under turbulence conditions. Results reveal more than 6dB of power penalty with the aged OF route for 100Gbps systems. In addition, there is a 0.8dB power penalty due to the strong seasonal induced PMD fluctuations. The influence of scintillations in terms of Rytov variance for the FSO link is also investigated for weak to moderate turbulence. Finally, we derive an expression for the long-term mean PMD value determined over one-month measured frequency response.

SOA Based Photonic Integrated WDM Cross-Connects for Optical Metro-Access Networks

We present a novel optical metro node architecture that exploits the Wavelength Division Multiplexing (WDM) optical cross-connect nodes for interconnecting network elements, as well as computing and storage resources. The photonic WDM cross-connect node based on semiconductor optical amplifiers (SOA) allows switching data signals in wavelength, space, and time for fully exploiting statistical multiplexing. The advantages of using an SOA to realize the WDM cross-connect switch in terms of transparency, switching speed, photonic integrated amplification for loss-less operation, and gain equalization are verified experimentally. The experimental assessment of a 4 × 4 photonic integrated WDM cross-connect confirmed the capability of the cross-connect chip to switch the WDM signal in space and wavelength. Experimental results show lossless operation, low cross-talk &lt;−30 dB, and dynamically switch within few nanoseconds. Moreover, the operation of the cross-connect switch with multiple WDM channels and diverse modulation formats is also investigated and reported. Error-free operation with less than a 2 dB power penalty for a single channel, as well as WDM input operation, has been measured for multiple 10/20/40 Gb/s NRZ-OOK, 20 Gb/s PAM4, and data-rate adaptive DMT traffic. Compensation of the losses indicates that the modular architecture could scale to a larger number of ports.

Adaptation of Mode Filtering Technique in 4G-LTE Hybrid RoMMF-FSO for Last-Mile Access Network

This paper demonstrates a hybrid radio over multimode fibre and free space optics (RoMMF-FSO) system that can be used to extend the transmission range of the fourth generation long-term evolution (4G-LTE) signal in access networks. A single mode filtering technique (SMFT) is used to enhance 4G-LTE performance. The proposed scheme is evaluated in terms of the system transfer function, laser beam profile, and error vector magnitude (EVM). We show that using SMFT increases the RoMMF-FSO system bandwidth by 2 GHz and improves the received optical power by 13.6 dB. Moreover, the proposed system enhances the EVM by 4%. The measured results show that by using a 1 km MMF instead of a 1 km SMF will marginally increase the measured EVM from ∼6.6% to ∼7% with a 0.2 dB power penalty with respect to the LTE EVM limit of 12.5% as is specified for 16-quadrature amplitude modulation. The proposed system is validated practically under atmospheric turbulence conditions to mimic the outdoor environment. Measured EVM results are verified theoretically through transmitting LTE signals with turbulent using log-normal model. We also show that for a FSO link span of 500 m to meet the EVM target of 12.5% the SNR power penalties are ∼2 dB and ∼11 dB for Rytov variance of 1.2 × 10−4 and 0.1, respectively, compared with no turbulence.

Monolithically Integrated Wavelength Converter for Intra-Data Center Routing Applications

A wideband wavelength converter based on cross gain modulation has been demonstrated by using a monolithically integrated semiconductor optical amplifier and 12-channel distributed feedback lasers. Error-free operation unicast wavelength conversion of 10-Gb/s non-return-to-zero ON–OFF keying data payload has been demonstrated with 3-dB power penalty and around 0.5 dB extra penalty for transmission over 2-km single-mode fiber for intra-data center application. Performance of the wavelength converter multicasting operation of two channels has also been assessed by controlling the distributed-feedback lasers. Experimental results show 4.5 dB power penalty after multicast wavelength conversion. The wavelength conversion of uni-/multi-casting operations would provide a compact, low power, and low-cost solution for wavelength routing-based intra-data center networking applications.

Silicon mode multiplexer processing dual-path mode-division multiplexing signals.

Great strides have been made in the optical interconnect for a mode-division multiplexing (MDM) system. The mode multiplexer/demultiplexer (MUX/DEMUX) is the crucial component for the MDM system. We propose and demonstrate a scheme that is different from the reported multiplexing schemes, mode MUX that tackles dual-path MDM signals simultaneously and avoids mode conflict. The performance is experimentally demonstrated by integrating the proposed MUX and DEMUX into a MDM link, with ∼1 dB of insertion loss and ∼18 dB extinction ratio over the C-band. Single-wavelength, non-return-to-zero on-off keying signals at 10Gb/s carried on dual-path different modes are successfully processed with open and clear eye diagrams, and <1 dB power penalty are obtained.

Flexible and scalable wavelength multicast of coherent optical OFDM with tolerance against pump phase-noise using reconfigurable coherent multi-carrier pumping.

Recently the ever-growing demand for dynamic and high-capacity services in optical networks has resulted in new challenges that require improved network agility and flexibility in order for network resources to become more "consumable" and dynamic, or elastic, in response to requests from higher network layers. Flexible and scalable wavelength conversion or multicast is one of the most important technologies needed for developing agility in the physical layer. This paper will investigate how, using a reconfigurable coherent multi-carrier as a pump, the multicast scalability and the flexibility in wavelength allocation of the converted signals can be effectively improved. Moreover, the coherence in the multiple carriers prevents the phase noise transformation from the local pump to the converted signals, which is imperative for the phase-noise-sensitive multi-level single- or multi-carrier modulated signal. To verify the feasibility of the proposed scheme, we experimentally demonstrate the wavelength multicast of coherent optical orthogonal frequency division multiplexing (CO-OFDM) signals using a reconfigurable coherent multi-carrier pump, showing flexibility in wavelength allocation, scalability in multicast, and tolerance against pump phase noise. Less than 0.5 dB and 1.8 dB power penalties at a bit-error rate (BER) of 10-3 are obtained for the converted CO-OFDM-quadrature phase-shift keying (QPSK) and CO-OFDM-16-ary quadrature amplitude modulation (16QAM) signals, respectively, even when using a distributed feedback laser (DFB) as a pump source. In contrast, with a free-running pumping scheme, the phase noise from DFB pumps severely deteriorates the CO-OFDM signals, resulting in a visible error-floor at a BER of 10-2 in the converted CO-OFDM-16QAM signals.

Cost effective wavelength reused MDM system for bidirectional mobile fronthaul.

In this paper, we propose a cost-effective wavelength-reused mode-division-multiplexing (MDM) system for high speed symmetrical bidirectional mobile fronthaul application. At the base band unit (BBU) pool, one of the spatial modes is used to transmit signal carrier while the others are used for downstream (DS) signal channels. At the remote radio unit (RRU) side, the signal carrier is split and reused as modulation carrier for all the upstream (US) signal channels after mode demultiplexing. Thanks to the low mode crosstalk characteristic of the mode multiplexer/demultiplexer (MUX/DEMUX) and few-mode fiber (FMF), the signal carrier and each signal channel can be effectively separated. The spectral efficiency (SE) is significantly enhanced when multiple spatial channels are used. Compared with other wavelength reused scheme in which the downstream and upstream be modulated in orthogonal dimension, the modulation format of both directions are independent in the proposed wavelength reused MDM system. Therefore, it can easily achieve symmetrical bidirectional transmission without residual re-modulation crosstalk. The proposed scheme is scalable to multi-wavelength application when wavelength MUX/DEMUX is utilized. With the proposed scheme, we demonstrate a proof of concept intensity modulated 4 × 25-Gb/s 16-QAM orthogonal frequency division multiplexing (OFDM) transmission over 10-km FMF using low modal-crosstalk two-mode FMF and MUX/DEMUX with error free operation. The downstream receiver sensitivity is -21 dBm while the upstream receiver sensitivity is -18 dBm for bidirectional transmission. Due to the Rayleigh backscattering and other spurious reflections, the upstream suffers 2 dB power penalty compared with unidirectional transmission without downstream. To mitigate bidirectional transmission impairments, we propose a simple and effective method to suppress Rayleigh backscattering by shifting the downstream subcarrier frequency. A receiver sensitivity improvement of up to 2.5 dB is achieved for upstream with different downstream power.

Performance Degradation of Integrated Optical Modulators Due to Electrical Crosstalk

In this paper, we investigate electrical crosstalk in integrated Mach-Zehnder modulator arrays based on n-doped InP substrate and show that it can be the cause for transmitter performance degradations. In particular, a common ground return path between adjacent modulators can cause high coupling noise up to −20 dB which leads to system power penalties of more than 10 dB at 10 Gb/s OOK modulation. Furthermore, we demonstrate that electrical crosstalk is significantly reduced in the absence of the shared ground and that it varies with modulator separation distance. Experimental results are shown that indicate a crosstalk tolerance of −40 dB for 1 dB power penalty at 10 Gb/s.

Switchable in-line monitor for multi-dimensional multiplexed photonic integrated circuit.

A flexible monitor suitable for the discrimination of on-chip transmitted mode division multiplexed (MDM) and wavelength division multiplexed (WDM) signals is proposed and fabricated. By selectively extracting part of the incoming signals through the tunable wavelength and mode dependent drop filter, the in-line and switchable monitor can discriminate the wavelength, mode and power information of the transmitted signals. Being different from a conventional mode and wavelength demultiplexer, the monitor is specifically designed to ensure a flexible in-line monitoring. For demonstration, three mode and three wavelength multiplexed signals are successfully processed. Assisted by the integrated photodetectors (PDs), both the measured photo currents and eye diagrams validate the performance of the proposed device. The bit error ratio (BER) measurement results show less than 0.4 dB power penalty between different modes and ~2 dB power penalty for single wavelength and WDM cases under 10-9 BER level.

Crosstalk analysis of ring resonator switches for all-optical routing.

Optical switches based on ring resonator cavities were fabricated by a silicon photonics foundry process and analyzed for optical crosstalk at various data rates and channel spacings. These devices were compared to commercial bandpass filters and at 20Gb/s, 0.5dB power penalty is observed due to spectral filtering for bit error ratio threshold of 1 × 10-9. Concurrent modulation at 20Gb/s with a channel spacing as narrow as 40GHz shows error-free transmission with 1dB power penalty as compared to wider channel spacing for the ring-based switch.

A WDM/TDM Access Network Based on Broad T-Band Wavelength Resource Using Quantum Dot Semiconductor Devices

A wavelength-division multiplexing/time-division multiplexing (WDM/TDM) network exploiting the broad wavelength range of T-band (1000-1260 nm) is proposed. The network is based on different wavebands of downlink and uplink channels, with each transmitter consisting of a wavelength tunable laser utilized for more than one user through wavelength tuning. The downlink section of the network also introduces interconnecting optical lines and semiconductor optical amplifiers (SOAs), pairing up each of the downlink wavebands to provide structural redundancy by allowing transmitters to be shared between the paired wavebands through TDM. Quantum dot (QD)-based wavelength tunable lasers and SOAs were fabricated for a preliminary demonstration experiment. Error-free transmission with less than 1 dB of power penalty was obtained for a wavelength range of 24.2 nm (5.6 THz). The QD-SOAs exhibited a decent rise and fall time, as well as extinction ratios throughout the wavelength range, which enable it to be used as path switches.

Simple Scheme for PDM-QPSK Payload Generation in an Optical Label Switching Network

We propose and experimentally demonstrate a novel and simple scheme for dual-polarization vector pay-load generation in an optical label switching network. The 8 Gbaud quadrature-phase-shift-keying (QPSK) payload, generated by one intensity modulator with signal precoding and optical carrier suppression, is first polarization-multiplexed by a polarization multiplexer, then combined with a 2.5 Gb/s label before being transmitted over 20 km single-mode fiber, and finally separated from the label and recovered by the receiver with less than 2 dB power penalty. The proposed scheme can facilitate label insertion and is suitable for an optical label switching network with a simple architecture.

Raman Crosstalk Suppression in NG-PON2 Using Optimized Spectral Shaping

In this paper, we present an innovative proposal to mitigate the effect of Raman crosstalk in the interaction between NG-PON2 TWDM-PON channels and legacy RF-video by electrical spectral shaping on the TWDM NRZ data stream obtained through electrical FIR filtering. We find an analytical and closed form solution on FIR filter taps that, for a given target spectral shaping, gives the absolute minimum amount of intersymbol interference on the TWDM data. Then, we propose techniques to implement these FIR filters with reasonable system complexity. We found that our proposed technique is applicable to all classes (up to E2 PON, +11 dBm transmitted downstream power) with less than 0.5 dB power penalty on TWDM channels even in the worst-case scenario of RF-video still based on analog channels on the lower part of the spectrum.

Mode division multiplexing using an orbital angular momentum mode sorter and MIMO-DSP over a graded-index few-mode optical fibre.

Mode division multiplexing (MDM)– using a multimode optical fiber’s N spatial modes as data channels to transmit N independent data streams – has received interest as it can potentially increase optical fiber data transmission capacity N-times with respect to single mode optical fibers. Two challenges of MDM are (1) designing mode (de)multiplexers with high mode selectivity (2) designing mode (de)multiplexers without cascaded beam splitting’s 1/N insertion loss. One spatial mode basis that has received interest is that of orbital angular momentum (OAM) modes. In this paper, using a device referred to as an OAM mode sorter, we show that OAM modes can be (de)multiplexed over a multimode optical fiber with higher than −15 dB mode selectivity and without cascaded beam splitting’s 1/N insertion loss. As a proof of concept, the OAM modes of the LP11 mode group (OAM−1,0 and OAM+1,0), each carrying 20-Gbit/s polarization division multiplexed and quadrature phase shift keyed data streams, are transmitted 5km over a graded-index, few-mode optical fibre. Channel crosstalk is mitigated using 4 × 4 multiple-input-multiple-output digital-signal-processing with <1.5 dB power penalties at a bit-error-rate of 2 × 10−3.