Directly Modulated Laser Research Articles

Compensation of nonlocal effects induced by the extinction ratio of pump pulses in Brillouin optical time-domain analysis sensors.

We demonstrate a technique to compensate the nonlocal effects that appear in Brillouin optical time-domain analysis sensors when pump pulses with limited extinction ratio are deployed. These recently discovered nonlocal effects are originated in the interaction between the probe wave and the pulse pedestal. Hence, their compensation method is based on deploying a modulation (dithering) of the optical frequency of the probe and pulse pedestal waves that provides a reduction of the effective interaction length between them. This is implemented by taking advantage of the chirp associated to the direct current modulation of a semiconductor laser used as common source for both waves. The net effect of this procedure is that the probe and pulse pedestal waves display efficient Brillouin interaction just at correlation peaks along the fiber where the frequency difference between both waves remains constant. Proof-of-concept experiments in a 25-km sensing link demonstrate the performance of the technique, where large errors of more than 10 MHz in the measurement of the Brillouin frequency shift are completely compensated by introducing a sinusoidal dithering to the laser source.

Cost Efficient 4 × 2.5 Gb/s Transparent WDM Ring Network using DML and MetroCor Fiber for Long Reach Applications

Abstract In this paper, we analyze the performance of the 4 × 2.5 Gb/s wavelength division multiplexing (WDM) transparent metro ring network comprising of one network node and eight access nodes. Here, the power requirement of 360 km ring network has been evaluated using the optical signal to noise ratio (OSNR), Q-factor, inter-symbol interference (ISI) and power penalty. The metro network utilized the cost-effective directly modulated laser (DML) and a negative dispersion fiber called MetroCor in order to give enhanced performance in terms of Q-factor. The power requirement of the network is optimized for the signals over SMF-28 and MetroCor fibers. Also, the comparison is drawn between uncompensated and compensated (with -post and -symmetrical) SMF-28 fibers in the metro environment. The excellent value of Q-factor is obtained at all the nodes. Also, the results show higher OSNR values in the metro environment. Although, smaller power penalties are detected for nodes closer to the network node, but distant nodes require higher power for better transmission.

Proposal of external modulation scheme for fiber-optic correlation-domain distributed sensing

All the previous configurations of optical correlation-domain reflectometry (OCDR) have been based on the direct modulation of laser driving current. However, some lasers are not designed for high-speed large-amplitude modulation, and the modulation amplitude and frequency are mutually dependent. In this work, to mitigate these drawbacks, we propose an external modulation scheme for OCDR. After confirming its basic operation, we show that a non-specially designed laser can be employed. We also clarify that the interdependence of the modulation amplitude and frequency is negligibly small in this scheme, which is not the case for the conventional direct modulation scheme.

An Electronic Dispersion Compensation Transceiver for 10- and 28-Gb/s Directly Modulated Lasers-Based Optical Links

This paper presents an electronic dispersion compensation (EDC) transceiver to encompass extended reach of both 10- and 28-Gb/s directly modulated lasers (DMLs). In DML-based links, direct modulation of laser diodes induces power-dependent frequency chirp on the generated optical signals, which causes severe chromatic dispersion as well as pattern dependence on the received signal after optical fiber transmission. Such time-varying nature of the dispersion makes conventional equalization methods ineffective for compensation. The proposed EDC-based clock and data recovery (CDR) IC overcomes the chirp-induced dispersion via a linear equalizer followed by a double-sampling decision feedback equalizer (DFE) at the receiver and a pattern-dependent pulse shaping filter at the transmitter. The chirp-managed EDC improves the receiver sensitivity at a bit error rate (BER) of 10−6 by 1.3 dB in a 3-km link using a DML at a modulation rate of 28 Gb/s, where transient chirp is dominant, and by more than 7 dB in a 105-km link at 10 Gb/s, where adiabatic chirp is dominant. The chip prototype is fabricated in a 40-nm CMOS process and packaged in a flip-chip ball grid array (BGA). The single-chip IC containing dual-lane transceivers and a global clock generator (CG) occupies an active area of 2.8 mm2. The test chip consumes 236 mW, wherein a single-lane transceiver consumes 105.5 mW and a global CG consumes 25 mW from a 0.9-V supply at 28 Gb/s.

Package-level passive equalization technology enabling DML-based 112 Gbps/λ PAM4 transmission

We demonstrate a package-level passive equalization technology in which the wire-bonding-induced resonance effect is used to compensate for the limited gain strength within the Nyquist frequency. The corresponding gain strength under various inductance and capacitance combinations could be quantitatively determined using a numerical simulation. With the increase in the Nyquist frequency, the capacitance shows a greater effect on the gain strength than the inductance. Therefore, the parasitic capacitance should be decreased to realize the desired gain strength at a higher Nyquist frequency. With this equalization technology, gain strength of 5.8 dB is obtained at 22 GHz, which can compensate for the limited bandwidth for the 112 Gbps pulse amplitude modulation (PAM4) signal. The experimental results show that 112 Gbps/λ PAM4 transmission based on a directly modulated laser (DML) module can be realized with a bit error rate of 1 × 10−3 at a received optical power of 3 dBm.

Performance comparison of DML, EML and MZM in dispersion-unmanaged short reach transmissions with digital signal processing.

In this paper, transmission performances of directly modulated laser (DML), electro-absorption modulated laser (EML) and Mach-Zehnder modulator (MZM) are experimentally compared in dispersion-unmanaged high-speed transmission systems with digital signal processing (DSP). We show that, although the DML based transmitter is often believed to be less favorable in C-band high-speed transmissions, it exhibits superior performance over the other two transmitters when either linear or nonlinear digital signal processing is adopted. By theoretical and experimental analysis, we reveal that the superiority of DML can be attributed to the compensation of fiber power fading by its inherent adiabatic chirp as well as the mitigation of chirp induced distortions by the linear or nonlinear equalization. Experimental results of 56Gb/s 4-level pulse amplitude modulation (PAM4) signals under various equalization schemes including linear feedforward equalization, simplified nonlinear Volterra equalization and partial response signaling are presented. Particularly, we show that for DML a 40km transmission distance can be achieved to satisfy the extended range-4 (ER4) Ethernet interconnect using a simplified Volterra equalizer, and a 20km transmission distance can be supported using a linear equalizer. In contrast, for MZM and EML, the achievable transmission distances are respectively 20km and 15km using the Volterra equalizer, respectively, and 15km and 10km using linear equalizer, respectively. Moreover, we show that even using the combination of the Volterra equalizer and partial response signaling, the transmission distances of MZM and EML based systems are limited to 30km and 20km.

Advanced Digital Signal Processing for Reach Extension and Performance Enhancement of 112 Gbps and Beyond Direct Detected DML-Based Transmission

112 Gbps per wavelength, amplification free Four-level Pulse Amplitude Modulation (PAM-4) transmissions are experimentally demonstrated with advanced digital signal processing (DSP) algorithms. Two DSP architectures are investigated for Directly Modulated Lasers (DMLs): 1) the regular DSP with Feed Forward Equalization (FFE) and Decision Feedback Equalization (DFE), and 2) the enhanced DSP with Maximum Likelihood Sequence Estimation (MLSE) equalizer. The experimental results show that with advanced DSP technologies the conventional DML can achieve 40 km transmissions with Bit Error Rate (BER) under 2.4 × 10−4 that meets RS (544, 514) Forward Error Correction (FEC) requirement; with regular-DSP the advanced DML can achieve 40 km transmissions with BER under 3.8 × 10−3, with stronger FEC. 112 Gbps and beyond per lambda DML based transmission is a promising low power, low-cost solution for 800 GbE or 1.6 TbE Ethernet.

4 × 128-Gb/s PDM-DMT signal transmission over 1440-km SSMF with high phase noise tolerance.

In this paper, we propose an intensity modulation and coherent detection scheme with high phase noise tolerance for polarization division multiplexing (PDM) discrete multi-tone (DMT) signal transmission by employing Kramers-Kronig (KK) detection and digital carrier regeneration (DCR). At the transmitter side, DMT signal is modulated by a Mach-Zehnder modulator (MZM) setting bias around the null point and transmitted with the suppressed optical carrier. At the receiver side, a directly modulated lasers (DMLs) locating at the edge of DMT signal is used as the local oscillator (LO) for coherent detection. For signal recovery, KK detection is first used to reduce the signal to signal beating noise. Digital optical carrier is then regenerated by the DCR scheme and the DMT signal could be recovered by enveloping detection with the regenerated digital carrier, which can achieve high laser line-width tolerance and mitigate the residual phase noise caused by KK detection. To verify the effectiveness of the proposed scheme, we compare the KK and DCR based receiver digital signal processing (DSP) with conventional receiver DSP by using both DMLs with larger linewidth (~10 MHz) or external cavity lasers (ECLs) with smaller linewidth (less than 100 kHz) as optical carrier and LO. The results show that KK and DCR can mutually improve the system performance with ECLs as optical carrier and LO. Moreover, it is shown that the signal using DMLs cannot be recovered without the DCR method due to the high laser line-width of DML. Finally, we successfully demonstrate 4 × 128-Gb/s KK and DCR based PDM-DMT signal transmission over 1440-km SSMF by employing DMLs as optical carrier and LO.

Adaptive Blind CSO Cancellation Technique for RoF Systems Implemented by Using DMLs

We report on the adaptive cancellation technique of the dispersion-induced composite second-order (CSO) distortions for use in radio-over-fiber (RoF) systems implemented by using directly modulated lasers (DMLs). The proposed technique utilizes the magnitude of the CSO distortions measured at outside the signal’s bandwidth as a cost function of the steepest descent algorithm. As a result, this technique can blindly cancel out the CSO distortions without the necessity of measuring any device and fiber parameters used in the RoF system. The effectiveness of the proposed technique is confirmed in the RoF link implemented by using a 1.55- $\mu \text{m}$ DML and is capable of transporting 24 100-MHz filtered-orthogonal-frequency-division-multiplexing signals in 64 quadrature amplitude modulation format.

High tolerance against chirp induced distortions in PAM4-based digital mobile fronthaul by sample bits interleaving.

Directly modulated lasers (DMLs) and electro-absorption modulated lasers (EMLs) are key transmitter options in future short-haul networks. However, both of them suffer from frequency chirp, which incurs nonlinear distortions, especially to high order modulation signals. In this paper, we investigate their application in PAM4-based digital mobile fronthaul and propose a scheme to remarkably improve the fidelity of radio signal. We first give a detailed study of the BER distribution of DML/EML based PAM4 signals and find that the BER of the second bit is much higher than that of the first bit in both systems. Accordingly, we propose to adopt sample bit interleaving to reduce the radio signal distortions caused by sample bit errors. Experimental results of 56Gbps I/Q data transmission reveal that, in a DML-based transmission system, the proposed scheme respectively leads to up to 8dB and 13dB EVM reduction to accommodate 33 × 100MHz 1024QAM OFDM signals and 64QAM OFDM signals in 10km and 20km cases. As well as in an EML-based transmission system, 14dB EVM reduction is achieved in 10km to finally accommodate 33 × 100MHz 256QAM OFDM signal.

Measurement error induced by the power-frequency delay of the light source in optical correlation-domain distributed Brillouin sensors.

Direct current modulation of a semiconductor laser is a key method adopted in most optical correlation-domain distributed Brillouin sensors such as Brillouin optical correlation-domain analysis (BOCDA) and reflectometry (BOCDR) for localizing the sensing position by synthesis of an optical coherence function. We report that distributed measurement of Brillouin frequency by the BOCDA or BOCDR system can suffer significant distortion caused by phase delay between output power and frequency variations of the modulated light source. We calculate characteristics of the distortion by numerical simulations, and compare the results with experimental data obtained by a BOCDA system for confirmation. Our results show that a maximum error of more than 30MHz can occur in Brillouin frequency measured under ordinary operating conditions with MHz-order direct current modulation.

Enhanced Multi-Level Signal Recovery in Mobile Fronthaul Network Using DNN Decoder

A deep neural network (DNN) decoder which combines the function of both equalization and decoding is proposed and experimentally demonstrated for mobile fronthaul (MFH) transmission. This DNN consists of one input layer, one output layer, and several hidden layers. Adamax algorithm is implemented for finding the global minima while dropout mechanism and early stopping are utilized to avoid overfitting. The DNN accepts the received samples as the input and output the decoded samples directly to recover the transmitted samples. Using this DNN decoder, a record-high data-rate transmission-distance product at 1800-Gb/s $\cdot $ km based on the directly modulated laser (DML) with intensity-modulation direct-detection is obtained. Besides, the PAM8 modulation with 3-bps/Hz spectral efficiency is implemented. Due to the smaller source spectrum bandwidth compared with traditionally widely used PAM4 and OOK at a certain data rate, the power fading limited transmission distance is extended by 1.5 and three times, respectively. The DML is a low-cost device and this simple transmission system eliminates the need of single-side-band modulation or dispersion compensation, which makes this system an ideal candidate for a low-cost enhanced MFH network.

Low-operating-energy directly modulated lasers for short-distance optical interconnects

We review recent developments in directly modulated lasers (DMLs) with low operating energy for datacom and computercom applications. Key issues are their operating energy and the cost for employing them in these applications. To decrease the operating energy, it is important to reduce the active volume of the laser while maintaining the cavity Q-factor or photon lifetime in the cavity. Therefore, how to achieve high-reflectivity mirrors has been the main challenge in reducing the operating energy. In terms of the required output power from the lasers, the required input power into the photodetector and the transmission distance determine the lower limit of laser active volume. Therefore, the operating energy and output power are in a trade-off relationship. In designing the lasers, the cavity volume, quantum well number, and optical confinement factor are critical parameters. For reducing the cost, it is important to fabricate a large-scale photonic integrated circuit (PIC) comprising DMLs, an optical multiplexer, and monitor photodetectors because the lower assembly cost reduces the overall cost. In this context, silicon (Si) photonics technology plays a key role in fabricating large-scale PICs with low cost, and heterogeneous integration of DMLs and Si photonics devices has attracted much attention. We will describe fabrication technologies for heterogeneous integration and experimental results for DMLs on a Si substrate.

Code reservation enabled PAPR reduction of digital CDM based channel aggregation for mobile fronthaul.

A code reservation technique is proposed to reduce the peak-to-average power ratio (PAPR) of digital code-division multiplexing (CDM) based channel aggregation for mobile fronthaul. We numerically investigate the relationship between the PAPR and the number of aggregated channels during the CDM based channel aggregation, and experimentally verify the transmission performance with the code reservation technique. The PAPR of aggregated signal, which is composed of 48x20MHz Long Term Evolution (LTE) signal mapped with 64 quadrature amplitude modulation (QAM), is reduced with the help of code reservation technique. The PAPR reduction enables larger optical modulation index (OMI) per channel at the linear operation region of a directly modulated laser (DML), leading to the optical signal-to-noise ratio (OSNR) improvement. After the transmission over 10km standard single mode fiber (SSMF), the receiver sensitivity can be improved by 4dB owing to the PAPR reduction.

Nonlinear Equalizer Based on Neural Networks for PAM-4 Signal Transmission Using DML

Nonlinear distortion from a directly modulated laser (DML) is one of the major limiting factors to enhance the transmission capacity beyond 10 Gb/s for an intensity modulation direct-detection optical access network. In this letter, we propose and demonstrate a low-complexity nonlinear equalizer (NLE) based on a machine-learning algorithm called artificial neural network (ANN). Experimental results for a DML-based 20-Gb/s signal transmission over an 18-km SMF-28e fiber at 1310-nm employing pulse amplitude modulation (PAM)-4 confirm that the proposed ANN-NLE equalizer can increase the channel capacity and significantly reduce the impact of nonlinear penalties.

Flexible generation of 28 Gbps PAM4 60 GHz/80 GHz radio over fiber signal by injection locking of direct multilevel modulated laser to spacing-tunable two-tone light.

To meet the ever-increasing bandwidth demands in the future broadband wireless networks, the millimeter-wave (mm-wave) frequency region is being actively perused, owing to its broad bandwidth and high frequencies. In this paper, a photonic mm-wave system is proposed and experimentally demonstrated based on the injection locking of a direct multilevel modulated laser to a spacing-tunable two-tone light. Since the mm-wave frequency of the generated signal is locked to the frequency spacing of the injected two-tone light, it shows better frequency stabilization than the schemes based on two free-running lasers. Moreover, by simply tuning the tone spacing, the mm-wave frequency could be easily re-configured, offering flexibility in the mm-wave signal generation. Instead of using complex and expensive optical modulators, the multilevel modulation on the mm-wave data carrier is implemented through the direct multilevel modulation of a laser and the injection locking. A 28 Gbps four-level pulse amplitude modulation (PAM4) is realized by biasing a 10 G-class laser at a current far from the threshold, providing a cost-effective and simple mm-wave generation scheme. In the experiment, a photonic approach to generating 28 Gbps PAM4 60 GHz/80 GHz mm-wave signals is experimentally demonstrated. A power penalty of less than 0.2 dB is observed for the filtered-out PAM4 signals with respect to the original PAM4. Besides, an ultra-low phase noise of up to -98 dBc/Hz is obtained for the mm-wave carriers after the injection locking. The proposed scheme possesses the flexibility and frequency stability of the mm-wave frequency, and also has low cost and implementation complexity.

Radial basis function neural network enabled C-band 4 × 50 Gb/s PAM-4 transmission over 80 km SSMF.

We propose a novel radial basis function neural network (RBF-NN)-based nonlinear equalizer (NLE) for the intensity modulation/direct detection (IM/DD) transmission. After optimizing input characteristics of the RBF-NN, we experimentally demonstrate C-band 4×50 Gb/s four-level pulse-amplitude modulation (PAM-4) transmission over 80km standard single-mode fiber (SSMF), using 18GHz direct-modulated lasers and dispersion compensation fiber. We demonstrate that the RBF-NN-based NLE outperforms the commonly used Volterra filter equalizer and recently proposed multilayer perceptron (MLP)-based NLE by about 4.5 and 1.5dB improvements of receiver sensitivity at the 7% forward error correction threshold, respectively. Furthermore, we identify that both the training stability and fitting ability of the RBF-NN-based NLE are better than those of the MLP-based NLE.

Experimental demonstration of 25-Gb/s downstream transmission using 10-Gbps optics for next-generation PONs

In this paper, we experimentally demonstrate a cost-effective 25-Gb/s downstream transmission of PON based on electro-absorption modulated laser (EML) and directly modulated laser (DML) both operating at 7-GHz. A 10-Gbps class avalanche photodiode (APD) is employed at the receiver. Adaptive feedforward equalizer (FFE) cooperating with faster-than-Nyquist NRZ (FTN-NRZ) technique, which includes FTN precoder and post decoder, are adopted to process the bandwidth limited signal. The experimental results indicate that pre-emphasis has a gain of 0.5 dB in EML based optical back-to-back transmission, and for 20-km standard single mode fiber (SSMF) link, the receiver sensitivities of -28.4 dBm and -26.5 dBm are achieved for EML and DML, respectively. According to the experimental results, the FTN-NRZ technique shows its superiority in receiver sensitivity, which is an alternative of cost-effective solution for future 100Gb/s TWDM-PON.

PAM-8 IM/DD Transmission Based on Modified Lookup Table Nonlinear Predistortion

In this paper, a modified lookup table (LUT) nonlinear predistortion method is proposed. With the modified LUT method, the number of patterns and the computational complexity can be reduced by half with a similar system performance comparing with normal LUT. We experimentally demonstrate an 8-ary pulse amplitude modulation (PAM-8) intensity modulation and direct detection (IM/DD) transmission over 15-km fiber using a directly modulated laser (DML) based on modified LUT. Around 1-dB receiver sensitivity improvement at bit-error ratio of 3.8 × 10 –3 is achieved by adopting a modified LUT over 15-km fiber transmission at 2 Gbaud based on a commercial DML. In addition, we also experimentally demonstrate Mach–Zehnder modulator based 40 Gbaud PAM-8 IM/DD transmission under back-to-back case, about 1-dB receiver sensitivity can also be improved.

4 × 25-Gb/s NRZ-OOK Signals Transmission Over a 160-km Single-Mode Fiber Using 10G-Class DML and Photodiode

In this paper, we have experimentally demonstrated the C-band 4 × 25-Gb/s non-return-to-zero on - off -keying (NRZ-OOK) signals over a 160-km single-mode fiber transmission based on 10G-class directly-modulated lasers and photodiodes. The excessive negative dispersion at the receiver side is used to equalize the frequency response of bandwidth-limited directly-modulated signal, therefore enables 25-Gb/s NRZ-OOK transmission based on 10G-class optical devices. By employing a multichannel chromatic dispersion compensation module in the receiver, a single optical device can be used to equalize the frequency response of multiple channels realizing low-cost and high-capacity signal transmission. The results shown in this paper reveal that our proposed scheme would be a promising candidate for a low-cost transition to 100 Gb/s (4 × 25 Gb/s) for the point-to-point metro network applications.