Electroabsorption Modulated Laser Research Articles

Simplified coherent optical network units for very-high-speed passive optical networks

Future passive optical networks (PONs) for 200Gb/s/λ, and beyond, pose a significant technological challenge. The use of coherent technology in access networks provides a great solution based on concepts from mature technologies to achieve the speeds needed for very-high-speed PON (VHSP). In this paper, we provide an overview of the currently demonstrated technologies and propose a possible simplified optical network unit (ONU) for time division multiplex PON (TDM-PON). The proposed ONU uses a single polarization heterodyne receiver, using either a balanced photodiode or a single-ended one and an electro-absorption modulated laser (EML)-based transmitter. The experimental demonstration using the proposed ONU in a bidirectional, symmetrical transmission over fiber distances of 20 km and 40 km shows the viability of the technology. The downstream direction achieves a power budget of 34.3/29.3 dB for 20 km and 33.9/29 dB for 40 km, for a balanced/single-ended receiver, whereas the upstream transmission achieves 29.3 dB for both scenarios.

212Gbps high-power EML for 800G artificial intelligence optical transmissions

We present a high-power, high-speed 212Gbps four-level Pulse Amplitude Modulation (PAM4) Electro-absorption Modulated Laser (EML) designed for 800G LR4 optical transmission and Artificial Intelligence (AI) applications. The Lan wavelength division multiplexing (LWDM) EML channels, operating at wavelengths of 1295.56 nm, 1300.05 nm, 1304.58 nm, and 1309.14 nm in 800G LR4 optical transceivers, exhibit clear eye openings even after 10km of transmission. Our 212Gbps PAM4 LWDM EMLs demonstrate high bandwidth, high extinction ratio, high power, and high energy efficiency, making them suitable for 10km transmission and environmentally friendly connectivity.

Highly Facet-reflection Immune 53GBaud EML for 800G Artificial Intelligence Optical Transceivers

We developed a facet-reflection immune 53GBaud electro-absorption modulated laser (EML) for 800G artificial intelligence (AI) optical network. An ultra-low anti-reflection (AR) coating reflectivity of 2x10-5 has been demonstrated for straight waveguide. Based on Hakki-Paoli method, we characterized the ultra-low AR using the ripple test technique. Such ultra-low AR is critical in achieving excellent eye pattern and optical transmission for 800G AI supercomputing.

106GBaud (200G PAM4) CWDM EML for 800G/1.6T Optical Networks and AI Applications

We report ultrahigh speed 106GBaud (200G PAM4) electro-absorption modulated laser (EML) for 800G and 1.6T optical transmission. Four CWDM EMLs of 1271, 1291, 1311 and 1331nm in 800G FR4 optical transceivers show clear eye diagram after 2km. Our 106GBaud EMLs show high bandwidth, high extinction ratio, low threshold current and high power, making it a suitable source laser for 800G/1.6T and AI applications.

A Decision Feedback Equalization Algorithm Based on Simplified Volterra Structure for PAM4 IM-DD Optical Communication Systems

A novel simplifying Volterra structure algorithm is proposed for an intensity modulation direct detection (IM-DD) optical fiber short distance communication system using the decision feedback equalization algorithm (DFE). Based on this algorithm, the signal damage for the four-level pulse amplitude modulation signal (PAM-4) is compensated, which is caused by device bandwidth limitation and dispersion during transmission. Experiments have been carried out using a 25 GHz Electro-absorption Modulated Laser (EML), showing that PAM-4 signals can transmit over 10 km in standard single-mode fiber (SSMF). The 112 Gbps and 128 Gbps signals can reach the error rate threshold of KP4-FEC (BER = 2 × 10−4) and HD-FEC (BER = 3.8 × 10−3), respectively. The simplified principle and process of the proposed Volterra-based equalization algorithm are presented. Experimental results show that the algorithm complexity is greatly reduced by 75%, which provides effective theoretical support for the commercial application of this algorithm.

Multi-band DFT-S 100 Gb/s 32 QAM-DMT transmission in intra-DCI using 10 G-class EML and low-resolution DAC.

In this paper, 100 Gb/s/λ 32 quadrature amplitude modulation discrete multi-tone (QAM-DMT) transmission using 10 G-class electro-absorption modulated laser (EML) and 4/5-bit digital-to-analog converters (DACs) are experimentally demonstrated to meet the requirement of intra-datacenter interconnection (intra-DCI). Unequal length multi-band (ULM) discrete Fourier transform spread (DFT-S) precoding is investigated to alleviate the distortion induced by the high peak-to-average power ratio (PAPR) of DMT. The results show that the required computational complexity of ULM DFT-S precoding with 2-bands (k1=256, k2=64) decreases sharply compared to the traditional DFT-S technique with only about 0.5 dB receiver sensitivity penalty. In addition, compared to the equal length multi-band (ELM) DFT-S precoding, the ULM DFT-S precoding can bring about 2.5 dB receiver sensitivity improvement with slight added computational complexity. With the assistance of ULM DFT-S precoding and noise shaping (NS) technique, the bit-error ratio (BER) of 100 Gb/s 32 QAM-DMT signal generated by 5-bit DAC over 2-km single-mode fiber (SMF) transmission can reach the hard-decision forward error correction (HD-FEC) threshold with received optical power (ROP) of -6.5 dBm, with only additional 39.9% multiplier and 33.7% adder.

Bidirectional Symmetrical 100 Gb/s/λ Coherent PON Using a Simplified ONU Transceiver

In this letter, a bidirectional symmetric 100 Gb/s/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda $ </tex-math></inline-formula> coherent passive optical network (PON) scheme with a simplified optical network unit (ONU) transceiver is proposed and experimentally demonstrated. In the proposed scheme, the downstream signal is generated in a dual-polarization coherent transmitter with Alamouti-coding at the optical line terminal (OLT) side and then received by a single-polarization heterodyne receiver at the ONU side. The upstream signal is generated in a low-cost C-band electro-absorption modulated laser (EML) at the ONU side and then received by a standard dual-polarization integrated coherent receiver at the OLT side. The experimental results show that more than 30 dB power budget is achieved after 25 km standard single mode fibre transmission for both downstream and upstream signal at 100 Gb/s/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda $ </tex-math></inline-formula> . The cost at the ONU side is maintained low as the required bandwidth of the optoelectrical device is >25 GHz and the local oscillator power at the ONU side can be as low as 4 dBm.

Simulation of an AlGaInAs/InP Electro-Absorption Modulator Monolithically Integrated with Sidewall Grating Distributed Feedback Laser by Quantum Well Intermixing

A novel AlGaInAs/InP electro-absorption modulated laser (EML) with a simple fabrication process is proposed, in which the electro-absorption modulator (EAM) has a 10 nm blueshift induced by quantum well intermixing (QWI) and is monolithically integrated with a sidewall grating distributed-feedback (DFB) laser working at 1.55 μm wavelength. The extent of the QWI process is characterized by a diffusion length. The quantum confined Stark effect (QCSE) is simulated in terms of extinction ratio (ER) and chirp for bias electric fields from 0 kV/cm to 200 kV/cm and for different amounts of intermixing. The results indicate that for a 150 µm-long EAM with a 10 nm blueshift induced by QWI, an ER of 40 dB is obtained at 2.5 V reverse bias with no penalty in chirp compared to an as-grown quantum well (QW) and the insertion loss at 0 V bias is 0.11 dB for 1.55 µm operation wavelength. The simulated –3 dB bandwidth of the electrical to optical power response is 22 GHz.

Digital mobile fronthaul based on delta-sigma modulation employing a simple self-coherent receiver

The coherent digital radio-over-fiber (DRoF) system is a promising candidate for future mobile fronthaul networks (MFNs) due to its high receiver sensitivity and excellent robustness against nonlinearities. However, conventional coherent receivers with complicated structure and heavy algorithms are too expensive and power-hungry for cost-sensitive MFN applications. In addition, currently deployed digital MFNs based on common public radio interface (CPRI) suffer from low spectral efficiency and high data rate. Towards these issues we propose a novel DRoF downlink scheme employing a simple self-coherent receiver. In baseband unit (BBU), the radio signal is converted to a digital bit stream by a band-pass delta-sigma modulator (BP-DSM), which can be simply recovered with the utilization of a band-pass filter at the receiver. In remote radio unit (RRU), an electro-absorption modulated laser (EML) acts as a low-cost coherent homodyne receiver in virtue of injection locking technique. In the experiment, the injection-locked operation of the DSM signal is successfully achieved, and two modified schemes are proposed for the DSM signal to increase the locking range with a tolerable sensitivity penalty. The experimental results demonstrate the superiority of our approach in two aspects: 1) the EML-based coherent receiver outperforms a PIN photodiode in terms of receiver sensitivity; 2) compared to the analog RoF system, a 5-dB improvement in loss budget is obtained when DSM is employed with the aid of a simple equalizer.

Low-Frequency-Drop of Electroabsorption Modulated Lasers and the Improvement With Partially Corrugated Grating Based DFB Lasers

Electroabsorption modulated lasers (EMLs) is sensitive to the residual facet reflection (RFR), which can cause a low-frequency drop (LFD) in the modulation response. The LFD may result in output waveform distortion, especially for high-data rate and multi-level modulation. We verify by simulation and experiments that the effect of LFD on the EML performance and propose to use LFD as one of the key performance indices for qualifying EMLs in high-speed optical transceivers. LFD can be suppressed by incorporating partial-corrugated-grating (PCG) in the laser section. With PCG-DFB structure, the EMLs can maintain high single-mode yield (SMY) and an excellent quality factor even with a strong modulator reflection from EAM section. This provides the robustness in applying the PCG-DFB to achieve a flat intensity modulation response or to enhance the output waveform under large signal modulation. By designing the PCG-EML to have about 60% of grating region, it can reach >80% of SMY and improve the average Q-value from the reduced LFD in the modulation response. The PCG-EML with reduced LFD can enhance the eye-opening through reduced waveform undershoot and overshoot for transmitting 56-Gbaud/s or beyond PAM-4 signals.

Mitigation of SOA-Induced Nonlinearity With the Aid of Deep Learning Neural Networks

Semiconductor optical amplifier (SOA) is a promising solution for future O-band optical amplification. However, the SOA-induced nonlinearity will affect the quality of the signals and cause bit errors. Nowadays, the neural network is a hot topic in communication networks and has been considered a practical algorithm for signals equalization. In this paper, we investigate the performance of the three types of neural networks: deep neural networks (DNN), convolutional neural network (CNN), and long short term memory (LSTM) neural networks for the compensation of the SOA-induced nonlinearity. We first compare the fitting ability of the three neural networks on the saturated response of SOA. Based on the simulation result, LSTM has the best nonlinear expressiveness for the SOA-induced nonlinearity. Subsequently, we do some experimental validations in an intensity modulation and direct-detection (IM/DD) system, where the 100 Gbit/s pulse-amplitude-modulation-4 (PAM4) signals are transmitted over 80 km at O-band using an 18 GHz electro-absorption modulated laser (EML). The bit-error-ratio (BER) results show that the cell states in LSTM play a crucial role in nonlinearity compensation. Furthermore, LSTM offers the best equalization performance and can achieve a receiver sensitivity of −18.0 dBm.

Gain-Coupled 4×56 Gb/s EML Array with Optimized Bonding-Wire Inductance

Gain-coupled 4 × 56 Gb/s electroabsorption modulated laser (EML) arrays are implemented by identical epitaxial layer (IEL) integration scheme. On-chip coplanar waveguide (CPW) electrodes are incorporated to reduce the parasitic capacitance and facilitate direct microwave probing of the device. Based on the circuit parameters extracted from the S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> data, the influence of bonding-wire inductance on the modulation performance of the EML is investigated. It is found that the modulation bandwidth can be enhanced by taking advantage of the bonding-wire induced resonance. Modulation bandwidth over 40 GHz as well as low microwave reflection has been experimentally confirmed by optimizing the bonding-wire inductance. Clear eye opening under 56 Gb/s nonreturn-to-zero (NRZ) modulation is demonstrated for the EML array with optimized bonding-wire length, and transmission through 10 km single-mode fiber (SMF) has been carried out.

Simultaneous frequency response measurement of electro-absorption modulation transceivers based on a self-referenced pilot operation.

An electro-optic method based on a self-referenced pilot operation is proposed for simultaneously characterizing electro-absorption modulation optoelectronic transceivers with a shared setup. Through inserting and extracting the self-referenced pilot, the frequency responses of electro-absorption modulated lasers (EMLs) and photodetectors (PDs) are independently obtained in a single measurement, and any extra optical-to-electrical or electrical-to-optical calibration is avoided. Specifically, the relative frequency response of the EML at fm is determined through the extracted difference-frequency pilot at fp (close to DC), realizing the low-frequency analysis for an EML. The relative frequency response of PD at 2fm+fp is obtained from the amplitude ratio of the extracted sum- and difference-frequency pilots at 2fm+fp and fp under the microwave driving signal at fm, verifying the doubled measuring frequency range. In the proof-of-concept demonstration, the frequency response of an electro-absorption modulation transceiver is measured up to 40 GHz. Thereinto, the frequency response of the EML is obtained by detecting the fixed low-frequency pilot of 10 kHz, and the frequency response of the PD is extracted with frequency-swept modulation to 20 GHz. The experiment results are compared with those obtained with the electro-optic frequency sweeping method to check for accuracy.

Beyond 200-Gb/s/λ DMT Signal Transmission With NGMI Optimization and Volterra Equalization

The band-limited and power fading effects are two main limitations to intensity-modulation direct-detection (IM-DD) transmission systems due to the low-cost transceivers and interactions from fiber dispersion. Entropy loading (EL) has the ability to squeeze out the last bit by continuous adjustment of entropy. However, it is a passively process to adapt to the channel response. Moreover, the normalized generalized mutual information (NGMI) target based EL may be overestimated because the practical binary forward error correction (FEC) codes have a gap to the theoretic limit, which may result in unsatisfied transmission capacity or post-FEC performance. To optimize the NGMI, we analyze the NGMI upper and lower boundaries defined by the achievable information rate (AIR) and FEC overhead (OH). We propose to use the AIR as target and optimize the NGMI with the FEC OH as constraint. We conduct an experiment to target the beyond 200-Gb/s discrete multi-tone signal transmission over 2-km or 10-km single mode fiber using a 32-GHz electro-absorption modulated laser (EML). However, we find that the channel response and generalized mutual information (GMI) cannot support the target AIR even with NGMI optimization. We propose a joint EL and second-order Volterra filter (VF) to alleviate the power fading effects and maximize the transmission capacity. To achieve the AIR target and use below 20% SD-FEC, we find the optimized NGMI is 0.92 to simultaneously meet the 2-km and 10-km transmission requirement. We successfully transmit beyond 200-Gb/s DMT signal over 2-km or 10-km single mode fiber with BER below 2.6 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−2</sup> in the EML-based direct-detection system.

Mitigation of optical multipath interference impact for directly detected PAMn system.

This paper investigates solutions for mitigating multipath interference (MPI) impact for intensity modulation direct detection systems, particularly for PAM4 systems. We propose a scheme that uses a high pass filter (HPF) at the receiver to remove the MPI induced carrier-carrier beat noise and therefore improve the MPI tolerance. The scheme has been verified via theoretical analysis, numerical simulation, and experiment. For a typical optical transmitter based on Mach-Zehnder modulator (MZM) without frequency chirp, over 5dB MPI tolerance can be improved for 106.4Gb/s PAM4 by using the proposed scheme. Investigation was also extended to electro-absorption modulated laser (EML) and directly modulated laser (DML) based optical transmitters. Results show that transient chirping in EML reduces the effectiveness of proposed MPI mitigation scheme. For EML with chirp factor smaller than 1.5, MPI tolerance gain of 2dB is still achievable. However, the scheme is ineffective for typical DML where the adiabatic chirp significantly broadened the laser linewidth.

20 GHz self‐oscillating parametric optical frequency comb generator using an electroabsorption modulated laser based dual‐loop optoelectronic oscillator

In this work, a self-oscillating parametric optical frequency comb generator is proposed and experimentally demonstrated. It consists of an electroabsorption modulated laser (EML) based dual-loop optoelectronic oscillation loop and a parametric frequency mixer. In the experiments, a 20 GHz radio-frequency signal is produced to drive the EML. The output pulses from the EML are amplified to work as pump seeds for the frequency mixer. In the mixer, a length of high non-linear fibre (HNLF) is firstly used to introduce chirps to the pump pulses, a section of single mode fibre is then employed for pulse compression, and finally another HNLF is applied for parametric frequency mixing. When the pump power is 28 dBm, a nine-line comb with a frequency spacing of 20 GHz and an optical signal-to-noise ratio >15 dB is generated. More comb lines can be obtained when another frequency mixer is cascaded.

Electroabsorption‐modulated laser as optical transmitter and receiver: status and opportunities

The rapid growth of digital services has led to a widespread deployment of opto-electronics that furnish the Internet as an efficient communication backbone. The electroabsorption-modulated laser (EML) is a representative example of a monolithic integrated electro-optic converter that has early become a commodity: it has been widely adopted in telecommunication networks in virtue of its cost- and energy-efficient light generation and modulation. This study reviews the state-of-the-art of EML applications. Despite its simplicity, the EML addresses numerous use cases that require either the transmission or the reception of optical signals, such as equaliser-free high-bandwidth intensity modulation/direct-detection links at low signal drive, analogue signal transmission with high signal integrity, spectral sculpting for dispersion-tolerant transmission and vector modulation. Full-duplex transceiver functionality in lieu of a pair of dedicated half-duplex sub-systems is eventually attained by combining transmission and reception. This strategy of significantly reducing the cost for a bidirectional communication engine will be discussed for coherent digital data and analogue radio-over-fibre transmission and optical ranging. The maturity of EMLs as coherent transceivers will be evidenced by a small penalty for realising full-duplex transmission and the accomplishment of homodyne detection, which obviates digital signal processing for the purpose of signal recovery.

25-Gb/s Laser Modulated EML With High Output Power

Using a conventional electro-absorption modulated laser (EML) device, with a dual-drive scheme, we show increased output power before modulator saturation effects degrade the signal quality. Both the laser and modulator section of the EML are driven simultaneously to generate a 25 Gb/s intensity modulated optical signal. With this dual-drive scheme we achieved a modulated output power of +7 dBm while maintaining a high extinction ratio (≥ 9 dB) and sufficient mask margin. Since the operating wavelength was 1301 nm, the optical path penalty was negligible after a 20 km fiber link. Compared with a conventional EML, in which the laser is continuously on and only the EAM is driven, we improved the mean modulated output power by 3.2 dB. With the proposed driving scheme, conventional simple and low-cost EMLs could be used to meet the requirements of high loss budget links such as those in passive optical network applications.

Coherent Ultra-Dense WDM-PON Enabled by Complexity-Reduced Digital Transceivers

Coherent technologies, along with digital signal processing (DSP), have revolutionized optical communication systems, significantly increasing the capacity of the fiber channel owing to transmission of advanced modulation formats and mitigation of propagation impairments. However, commercial solutions for high-capacity core networks are too complex and costly, and therefore hardly feasible, for access networks with high terminal density, where cost, power budget, and footprint are the main limiting factors. This article analyzes the key enabling techniques to implement a complexity-reduced coherent transceiver (CoTRX) by exploiting photonic integration, simplified optical modulation, low-cost DFB lasers, consumer electronics, and low-complexity DSP. Bulk optical modulators are replaced by direct amplitude-and-phase modulation of an integrated electro-absorption modulated laser (EML) with a smaller footprint, generating up to 8-ary modulation formats. Hardware-efficient DSP algorithms for the coherent transmitter and receiver, including pulse-shaping for direct phase modulation, differential detection for optical phase recovery, and digital pre-emphasis with enhanced tolerance to quantization noise, are investigated to face the challenges imposed by low-cost photonic and electronic devices, such as strong phase noise, wavelength drifts, severe bandwidth limitation, and low resolution data converters. Through numerical simulations and real-time experiments, the results indicate that this new class of CoTRX enables effective implementation of wavelength-to-the-user PON with dedicated 1.25–20 Gb/s per user, in an ultra-dense 6.25–25 GHz spaced WDM optical grid, with >30 dB loss budget, outperforming the current competing technologies for access networks.

Digital pre-distortion enabled chirp-optimized C-band EML in 129 Gbit/s PAM-8 signal transmission for DCI-campus

The use of a digital pre-distortion enabled C-band chirp-optimized electro-absorption modulated laser (EML) for Datacenter Interconnect Campus (DCI-Campus) is investigated for the first time. Pre-distortion techniques, specifically digital pre-equalization (Pre-EQ) and look-up-table (LUT), are applied to mitigate bandwidth limitation and nonlinear distortions, respectively. In an intensity modulation and direct-detection (IM/DD) system utilizing an EML, fiber dispersion induced frequency selective fading (FSF) is related to the chirp factor of the EML. To better suppress the FSF, the bias voltage of EML needs to be set to obtain a small positive or even a negative chirp factor, while the modulation nonlinearity appears. Nonlinear distortions introduced during FSF mitigation can be suppressed by the LUT scheme. With the digital pre-distortion enabled chirp-optimized EML in the IM/DD system, a 43 Gbaud PAM-8 signal is successfully transmitted over 5-km SMF-28, with a BER under the threshold for hard-decision feed-forward error correction (HD-FEC).