Directly Modulated Laser Research Articles

5 G new radio fiber-wireless converged systems by injection locking multi-optical carrier into directly-modulated lasers

The integration of fiber-optical wireless convergence with fifth generation new radio is crucial in building high-performance access networks. This approach not only provides high-transmission-rates but also ensures broad coverage, which is vital for future networks. Here we report fifth generation new radio fiber-wireless converged systems by injection locking multi-optical carrier into directly-modulated lasers. Data rates of 10 Gb/s, 20 Gb/s, and 40 Gb/s are achieved by direct modulation on directly-modulated lasers using 16-quadrature amplitude modulation-orthogonal frequency-division multiplexing signal. Through 25-km single-mode fiber, 1.5-km optical wireless, and 12-/22-/33-m millimeter-wave/sub-terahertz wireless integrated-media, 10-Gb/s/20-GHz, 20-Gb/s/60-GHz, and 40-Gb/s/100-GHz signals are transmitted with acceptably low bit error rates and error vector magnitudes, as well as distinct constellations. The successful transport of fifth generation new radio millimeter-wave and sub-terahertz signals at different carrier frequencies through fiber-wireless convergence demonstrates the potential of the system to meet the evolving requirement of next-generation communications.

Power stabilization of a terahertz-frequency quantum-cascade laser using a photonic-integrated modulator

We demonstrate a technique to stabilize the emission from a 3.4-THz quantum-cascade laser against power drifts, using a recently developed photonic integrated circuit (PIC) structure formed by coupling a racetrack resonator with a ridge waveguide. This structure enables a dynamic power-control range of ±15% and locking over >600 s using a proportional–integral control loop. The resonator yields a 50% weaker perturbation to the laser emission frequency when compared with direct laser modulation, and hence offers the prospect of simultaneous, quasi-independent control of power and frequency. Progress towards integrating the PIC with a precision micromachined rectangular metallic waveguide module has also been demonstrated, with power modulation of the principal laser emission line being observed during pulsed operation.

Weight-adaptive joint mixed-precision quantization and pruning for neural network-based equalization in short-reach direct detection links.

Neural network (NN)-based equalizers have been widely applied for dealing with nonlinear impairments in intensity-modulated direct detection (IM/DD) systems due to their excellent performance. However, the computational complexity (CC) is a major concern that limits the real-time application of NN-based receivers. In this Letter, we propose, to our knowledge, a novel weight-adaptive joint mixed-precision quantization and pruning approach to reduce the CC of NN-based equalizers, where only integer arithmetic is taken into account instead of floating-point operations. The NN connections are either directly cutoff or represented by a proper number of quantization bits by weight partitioning, leading to a hybrid compressed sparse network that computes much faster and consumes less hardware resources. The proposed approach is verified in a 50-Gb/s 25-km pulse amplitude modulation (PAM)-4 IM/DD link using a directly modulated laser (DML) in the C-band. Compared with the traditional fully connected NN-based equalizer operated with standard floating-point arithmetic, about 80% memory can be saved at a minimum network size without degrading the system performance. Quantization is also shown to be more suitable to over-parameterized NN-based equalizers compared with NNs selected at a minimum size.

End-to-end optimization of optical communication systems based on directly modulated lasers

The use of directly modulated lasers (DMLs) is attractive in low-power, cost-constrained short-reach optical links. However, their limited modulation bandwidth can induce waveform distortion, undermining their data throughput. Traditional distortion mitigation techniques have relied mainly on the separate training of transmitter-side pre-distortion and receiver-side equalization. This approach overlooks the potential gains obtained by simultaneous optimization of the transmitter (constellation and pulse shaping) and receiver (equalization and symbol demapping). Moreover, in the context of DML operation, the choice of laser-driving configuration parameters such as the bias current and peak-to-peak modulation current has a significant impact on system performance. We propose, to our knowledge, a novel end-to-end optimization approach for DML systems, incorporating the learning of bias and peak-to-peak modulation current to the optimization of constellation points, pulse shaping, and equalization. The simulation of the DML dynamics is based on the use of the laser rate equations at symbol rates between 15 and 25 Gbaud. The resulting output sequences from the rate equations are used to build a differentiable data-driven model, simplifying the calculation of gradients needed for end-to-end optimization. The proposed end-to-end approach is compared to three additional benchmark approaches: the uncompensated system without equalization, a receiver-side finite impulse response equalization approach, and an end-to-end approach with learnable pulse shape and nonlinear Volterra equalization but fixed bias and peak-to-peak modulation current. The numerical simulations on the four approaches show that the joint optimization of bias, peak-to-peak current, constellation points, pulse shaping, and equalization outperforms all other approaches throughout the tested symbol rates.

Dual wavelength signal generation with four wave mixing based on directly modulated laser

Dual wavelength (DW) lasers are crucial for producing petahertz (PHz) signals, which are higher value terahertz (THz) signals. The infrared and visible light sub-bands are valuable frequency range for a number of applications, including networking, multiple access techniques, next-generation mobile networks (6G), future wireless networks, and new PHz communication systems. The importune need to the high frequency signal such as THz and PHz signal for 5 and 6 next generation wireless cellular mobile networks motivate us to design an optical system to provide these signals rather than an electronic circuits. They are also beneficial for a variety of medical testing. The wavelength spacing ranging from 14 nm to 7 nm is proposed and demonstrated a dual-wavelength generation with FWM based on DML is realized at 1554 and 1548 nm. It is essential to make use of the fiber's non-linearity characteristics to ensure synchronization for continuous wave (CW) laser and directly modulated laser (DML) signals and the creation of the four wave mixing (FWM) components. With the help of OptiSystem version 14.0, the system is designed and investigated then, the desired THz signal is created as pulse amplitude modulation. The position of the FWM components within the frequency range between the spectrums of the two lasers affects the synthesis of the required signal. At the DML frequency equal to or below the CW Laser (1520 nm), neither FWM frequency components nor dual wavelength pulses exist. On the other hand, the FWM frequency components and dual wavelength pulses can only be seen when the DML frequency is greater than 1520 nm. The CW laser frequency is the main controlling factor in the generation THz signal. If the CW laser frequency is set at or below 1520 nm, the dual wavelength and THz signal are accessible. Because these frequencies are so far from the reference fiber frequency, it is impossible to produce dual wavelength pulses at frequencies higher than those mentioned above.

Optical parametric-assist frequency modulation for FMCW Lidar enabling double-range velocity measurement

Frequency-modulated continuous waves (FMCW) Lidar functioned as simultaneous measurement of velocity and distance has shown promising prospects in many applications. Direct modulation on semiconductor lasers is a common approach to achieve linear optical frequency scanning in FMCW Lidar for the simple configuration and easy implementation. However, the directly-modulated laser used to suffer from the limited scanning bandwidth, resulting in poor range resolution and limited velocity measurement range of FMCW Lidar. In particular, the issue becomes critical in the case of high velocity measurement over short distance. Here, we propose an optical parametric-assist frequency modulation (OPAFM) method to improve the velocity and distance measurement of FMCW Lidar simultaneously. Firstly, a chirp rate doubled idler light is generated via the four-wave mixing of a frequency-swept pump with a continuous-wave signal. The idler light then serves as a linear frequency swept laser source in the FMCW Lidar to double the range of velocity measurement and improve ranging resolution. We experimentally demonstrated the chirp rate improvement of frequency-swept light from 80.75 MHz/μs to 163.75 MHz/μs. To apply the idler light to the FMCW Lidar, the maximum measurable velocity can be doubled from 2.13 m/s to 4.41 m/s for a target at 5.2 m. Moreover, ranging measurement with improved resolution of 117 mm is also achieved, compared to the 234 mm ranging resolution by using the traditional FMCW Lidar solely. The maximum range can reach 41.54 m. The precision of velocity and distance measurement are 0.1 m/s and 3.5 cm, respectively. The method provides a promising solution to enhance the velocity and distance measurement of FMCW Lidar, enabling diverse applications in sensing and mapping.

Multistability, relaxation oscillations, and chaos in time-delayed optoelectronic oscillators with direct laser modulation.

We investigate the nonlinear dynamics of an optoelectronic oscillator that is implemented with a laser diode (LD) with time-delayed feedback. In this system, electrical-to-optical conversion is directly implemented using the direct modulation of the laser diode itself, instead of an electrooptical modulator as in conventional architectures. Moreover, we consider the cubic nonlinear saturation of the characteristic laser power-intensity (P-I) transfer function far above threshold, instead of its simplified piecewise linear counterpart. We perform the stability analysis of the oscillator, and we show that it displays a rich dynamics that includes quasi-harmonic, relaxation oscillations, and chaos. We also show that the oscillator is strongly hysteretic and displays a wide variety of multistable behaviors, including the rare case of bistability between chaotic attractors. Our analytical and numerical results are found to be in good agreement with the experimental measurements.

Demonstration of Reservoir Computing using Optoelectronic Oscillators with Direct Laser Modulation

Demonstration of Reservoir Computing using Optoelectronic Oscillators with Direct Laser Modulation

A Parabolic Waveform Generator Based on the Chirp Characteristics of a Directly Modulated Laser

Due to carrier dynamics, the modulated light field from a directly modulated laser (DML) has an intensity envelope with a certain frequency chirp. When the chirp is linearly mapped into intensity by a frequency discriminator such as an optical filter with a linear edge, the optical field presents a new signal determined by the multiplication operation between the envelope function and the chirp function. Under a triangular drive signal, this process can contribute dark, bright and frequency-doubled bright parabolic waveforms by properly adjusting the filter window. This method is verified by both a theoretical analysis and experimental demonstrations. It not only provides a low-cost and simple scheme to generate parabola signals, but also a new method for arbitrary waveform generation.

Tunable microwave frequency comb generation via periodic relaxation oscillation in directly modulated laser

In this paper, a novel and simple generation scheme based on a directly modulated laser (DML) is proposed for tunable microwave frequency combs (MFCs). When the input modulated RF power is intentionally controlled to be sufficiently high to drive the DML into its nonlinear operating region periodically, relaxation oscillations will be excited in the same period. The modulated light from the DML is subsequently detected by a photodetector, and a special kind of microwave signal source can be optically produced and shows up in the form of MFC in the frequency domain. In the proof-of-concept experiment, tunable MFCs are demonstrated with the bandwidth of more than 25 GHz, the comb spacing of 0.5 ∼ 3 GHz, and the flatness of less than ± 2 dB with the frequency from 3 GHz to 18 GHz. The bandwidth of the generated MFC is in accordance with the relaxation oscillation frequency of the DML. The comb spacing is strictly equal to the frequency of the modulated RF signal and thus flexibly adjustable. The carrier-to-noise ratio can be up to 48 dB, and the single-sideband phase noise is measured to be lower than − 90 dBc/Hz@1 kHz for MFCs with comb spacings of 0.7 ∼ 3 GHz.

A Simplified Volterra Equalizer Based on System Characteristics for Direct Modulation Laser (DML)-Based Intensity Modulation and Direct Detection (IM/DD) Transmission Systems

The nonlinear Volterra equalizer has been proved to be able to solve the problem of nonlinear distortion, but it has high computational complexity and is difficult to implement. In this paper, a simplified second-order Volterra nonlinear equalizer designed for intensity modulation/direct detection systems based on direct modulated laser is proposed and demonstrated, taking into account the characteristics of the system. It has been proved that the received signal of direct modulation laser/direct detection system can be expressed in Volterra series form, but its form is too complex, and the device parameters should also be considered. We re-derived it and obtained a more concise form. At the same time, we proposed a method to simplify the second-order Volterra nonlinear equalizer without relying on device parameters. The performance of the proposed Volterra nonlinear equalizer is evaluated experimentally on a 56 Gb/s 4-ary pulse amplitude modulation link implemented by using a 1.55 µm direct modulation laser. The results show that, compared with the traditional Volterra nonlinear equalizer, the receiver sensitivity of the equalizer is only reduced by 0.2 dB at most, but the complexity can be reduced by 50%; compared with diagonally pruned Volterra nonlinear equalizers, the complexity of the equalizer is the same, but the reception sensitivity can be improved by 0.5 dB.

MMW Signal Gain in DML-Based Microwave Photonic Links Under Large Signal Regime

This paper presents a theoretical and experimental analysis of the millimeter wave (mmW) signal gain observed in photonically generated signals by using carrier suppressed (CS) external modulation in a centralized network architecture while the data is transmitted over a directly modulated laser (DML) operating under different signal regimes. Measurements of the frequency response under small and large signal regimes are presented for the sake of comparison. Moreover, the mmW signal gain has been measured for different standard single mode fiber (SSMF) lengths in order to identify the conditions and impact of large signal regime operation. The results are also validated by the experimental transmission of a 25 MHz QPSK signal transmitted over 40 GHz since the error-vector-magnitude (EVM) is measured for different input electrical power values as well as several received optical power (RoP) levels. Moreover, the spurious-free dynamic range (SFDR) ratio is measured to compare the third-order intermodulation distortion (IMD3) between small and large signal regime. The quantitative results demonstrate that proper system conditions allow the exploitation of the mmW signal gain towards the future deployment of high energy efficient networks.

Nonlinear distortion mitigation of DML-based OFDM optical systems with non-orthogonal DFT-precoding.

We propose a non-orthogonal discrete Fourier transform (DFT) matrix precoding scheme for the mitigation of nonlinear distortion induced by the interaction between laser chirp and fiber dispersion in a directly modulated laser (DML)-based orthogonal frequency division multiplexing (OFDM) transmission system. Compared with conventional OFDM, the proposed method can decrease the peak-to-average power ratio (PAPR) and significantly reduce the nonlinear distortion without sacrificing spectral efficiency (SE). The cascaded binary-phase-shift-keying iterative detection (CBID) algorithm is used to eliminate the inter-carrier interference (ICI) that is purposely induced by the non-orthogonal precoding. The performance of the proposed scheme is experimentally evaluated, achieving ∼0.4-dB sensitivity improvement at the KP4-forward error correction (FEC) threshold over the T/2-spaced third-order Volterra nonlinear equalizer (VNLE). Meanwhile, compared to the VNLE, the reduction in computational complexity of one OFDM frame is 90% for multiplication and 88.32% for addition in this work.

Improvement on stability of direct modulation laser with integrated active feedback by partial corrugated gratings.

Researchers have developed a uniform grating DFB with an integrated active optical feedback waveguide (UG-AFDFB) to enhance the modulation speed of direct modulation lasers (DMLs) while reducing costs based on identical active layer designations. However, this design has difficulties in obtaining high single mode yield (SMY) and low relative intensity noise (RIN) as a result of the strong optical feedback caused by the integrated active feedback waveguide (AFW) and the random phase of the facet phase. In this paper, a partial corrugated grating DFB with an integrated active optical feedback waveguide (PG-AFDFB) is proposed to address this issue. Comparison of SMY, S21, RIN, modulation eye pattern, and frequency chirp parameters between UG-AFDFB and PG-AFDFB based on time-domain transmission line laser mode reveals that PG-AFDFB with an optimized grating couple parameter κ performs significantly better than UG-AFDFB under the same conditions. Furthermore, the performance of PG-AFDFB is not sensitive to the random phase of the rear facet phase. Even when κ ranges from 6000 /m to 12000 /m, the current in the AFW is between 0 mA and 20 mA, and the length of the AFW ranges from 50 µm to 100 µm; the SMY of PG-AFDFB remains above 80%.

CMOS Linear Laser Driver for Intermediate Frequency over Fiber (IFoF) Links

The main objective of the proposed linear laser driver (LLD) is to reduce signal distortion in an analog direct modulation laser configuration used for intermediate frequency over fiber links. This work draws on an open-loop configuration featuring two differential pair blocks in a cascade arrangement to achieve a bandwidth measurement of 415 MHz at the half-power point, a total harmonic distortion of 4.57% for a fundamental frequency of 100 MHz, and an amplitude of 100 mVpp. The LLD provides a gain of 12.3 dB for a differential output and an output impedance of 46 Ω. The design, layout, and integration correspond to the process design kit for TSMC 65-nm CMOS technology. Experimental results show the advantage over other previously reported laser drivers.

Methane detection with a near-infrared heterodyne phase-sensitive dispersion spectrometer at a stronger frequency modulation using direct injection-current dithering.

Heterodyne phase-sensitive dispersion spectrometer (HPSDS) retrieves the concentration of gas samples by measuring the refractive index fluctuations near the molecular resonance. Compared to previous HPSDS studies focusing on pure intensity modulation, it is attractive to investigate the performance of HPSDS sensor based on a distributed feedback (DFB) laser under conditions where frequency modulation is much higher than intensity modulation. In this work, we report the implementation of a near-infrared HPSDS for methane detection based on the direct modulation of a DFB laser. The performance of our HPSDS is assessed using the characteristic absorption peak of methane near 1653.7 nm. Long-time measurements show that our HPSDS has a detection limit (MDL) of 1.22 ppm at standard atmospheric pressure and room temperature. In the same experimental conditions, we have experimentally compared HPSDS to wavelength modulation spectroscopy (WMS) to evaluate the dynamical range, long-term stability, and precision limits of the two methods.

200 Gb/s Optical-Amplifier-Free IM/DD Transmissions Using a Directly Modulated O-Band DFB+R Laser Targeting LR Applications

We experimentally demonstrate an O-band single-lane 200 Gb/s intensity modulation direct detection (IM/DD) transmission system using a low-chirp, broadband, and high-power directly modulated laser (DML). The employed laser is an isolator-free packaged module with over 65-GHz modulation bandwidth enabled by a distributed feedback plus passive waveguide reflection (DFB+R) design. We transmit high baud rate signals over 20-km standard single-mode fiber (SSMF) without using any optical amplifiers and demodulate them with reasonably low-complexity digital equalizers. We generate and detect up to 170 Gbaud non-return-to-zero on-off-keying (NRZ-OOK), 112 Gbaud 4-level pulse amplitude modulation (PAM4), and 100 Gbaud PAM6 in the optical back-to-back configuration. After transmission over the 20-km optical-amplifier-free SSMF link, up to 150 Gbaud NRZ-OOK, 106 Gbaud PAM4, and 80 Gbaud PAM6 signals are successfully received and demodulated, achieving bit error rate (BER) performance below the 6.25%-overhead hard-decision (HD) forward-error-correction code (FEC) limit. The demonstrated results show the possibility of meeting the strict requirements towards the development of 200Gb/s/lane IM/DD technologies, targeting 800Gb/s and 1.6Tb/s LR applications.

Capacity-achieving symbol distributions for directly modulated laser and direct detection systems.

We investigate the capacity-achieving symbol distributions for directly modulated laser (DML) and direct-detection (DD) systems utilizing probabilistic constellation shaped pulse amplitude modulation formats. DML-DD systems utilize a bias tee to feed the DC bias current and AC-coupled modulation signals. Also, an electrical amplifier is commonly utilized to drive the laser. Thus, most DML-DD systems are subject to the constraints of the average optical power and peak electrical amplitude. We compute the channel capacity of the DML-DD systems under these constraints by using the Blahut-Arimoto algorithm to obtain the capacity-achieving symbol distributions. We also carry out experimental demonstration to verify our computation results. We show that the probabilistic constellation shaping (PCS) technique marginally increases the capacity of DML-DD system when the optical modulation index (OMI) is less than 1. However, the PCS technique allows us to increase the OMI larger than 1 without clipping distortions. As a result, we can increase the capacity of DML-DD system by using the PCS technique in comparison with the uniformly distributed signals.

Enhanced Modulation Bandwidth by Delayed Push–Pull Modulated DFB Lasers

The bandwidth of a distributed feedback (DFB) directly modulated laser (DML) is limited by its carrier-photon resonance (CPR) frequency. A viable approach to break the bottleneck is to introduce a photon-photon resonance (PPR), since the PPR can happen at a much higher frequency than the CPR. Among the many structures that can possibly generate the PPR, the dual-sectional push-pull modulated (PPM) DFB is of particular interest for its fabrication cost-effectiveness as no regrowth is required. The PPR in the PPM DFB, however, usually shows a rapid roll-off on both edges, which brings in an indentation on the lower frequency side of the PPR peak and, consequently, cuts off the bandwidth. To compensate for this dip, we introduce a detuned PPR and restart the CPR response by exploiting a time delay between the differential signals applied to the PPM DFB. Our simulation result shows that the broadened PPR peak and the restarted CPR response indeed mitigate the dip and effectively expand the PPM-DFB's bandwidth to approximately 50 GHz, a value double that of the conventional (single-sectional) DFB DML.

Experimental research of precoding techniques in a DML-based MB-OFDM UWBoF system

In this paper, an intensity-modulated direct-detection (IM/DD) multiband orthogonal-frequency-division-multiplexing ultra-wideband over fiber (MB-OFDM UWBoF) transmission system using a low-cost directly modulated laser (DML) is analyzed and experimentally verified. Precoding techniques and inter-symbol frequency-domain averaging (ISFA) algorithm are used in this experiment. Precoding techniques are used to suppress the noise introduced by DML and the power fading due to the imperfect frequency response of optoelectronic devices and frequency selective fading. Orthogonal Circular Matrix Transform (OCT) precoding and Discrete Fourier Transform (DFT) precoding were compared when the ISFA algorithm was used. In this scheme, for a single-mode fiber (SMF) link at a distance of 20 km, the 64 Quadrature amplitude modulation (QAM) mapped MB-OFDM UWBoF system using OCT precoding with ISFA and DFT precoding with ISFA scheme can improve the sensitivity of the optical receiver by 0.4 dBm and 0.284 dBm, respectively, compared to no precoding with ISFA at a BER of 3.8e-3. The minimum BER of the 64QAM mapped MB-OFDM UWBoF system scheme with OCT precoding and ISFA scheme is 2.56e-5, which achieves two orders of magnitude improvement compared to the BER threshold of the 7% hard-decision forward error correction (HD-FEC, 3.8e-3).