Multimode Vertical-cavity Surface-emitting Lasers Research Articles

Optical neurons to interconnect sensors and artificial brains in autonomous vehicles

AbstractWith the ever‐increasing need for faster data communication in cars, electrical interconnects are beginning to show bandwidth and electromagnetic compatibility limitations. The new IEEE 802.3cz ethernet physical layer has been developed specifically to meet these needs, based on a long‐wavelength multimode VCSEL (vertical cavity surface‐emitting laser) and mature bend‐insensitive multimode fibers. A major innovation is taking place that will make automotive‐grade, multigigabit optical transceivers possible.

Surface Photonic Crystal Engineering of a Multi-Mode VCSEL for a Bit-Loaded Broadband QAM-OFDM Data Link at 99 Gbit/s

Bit-loaded quadrature amplitude modulation-orthogonal frequency division multiplexing (QAM-OFDM) encoding and photonic-crystal-engineered multi-mode vertical-cavity surface-emitting lasers (MM-VCSELs) transmission performance are analyzed. Two different surface photonic-crystal designs are used to configure the core and cladding regions of MM-VCSELs, producing continuous-wave and digital-encoding outputs. These outputs are combined with the end-face-flattened OM5 multi-mode fiber (MMF) for 100 m short-reach transmission. The photonic-crystal (PhC) structure exhibits a spatial mode-filtering ability, supporting few or single-mode outputs from the MM-VCSEL. This helps reduce the modal dispersion during OM5-MMF transmission of the encoded data. Comparing the original MM-VCSEL with two different surface-photonic-crystal-configured MM-VCSELs, the allowable bit-loaded QAM-OFDM data rate can be increased from 60.7 (for the VCSEL without the PhC structure) to 85/65 Gbit/s (for the PhC VCSELs with 2-layer PhC structures in the cladding layer and the ones with a 1-layer PhC structure in the core layer and 2-layer PhC structures in the cladding layer, respectively) under back-to-back (BtB) encoding and enable the 100 m OM5-MMF transmission to increase from 58.5 (for the VCSEL without the PhC structure) to 81.2/64.6 Gbit/s (for the PhC VCSELs with 2-layer PhC structures in the cladding layer and the ones with a 1-layer PhC structure in the core layer and 2-layer PhC structures in the cladding layer), respectively. Furthermore, by comparing the 7°-titled and 0°-normalized vertical coupling conditions, it can be observed that the purely normalized vertical coupling can collect more output power, resulting in an improved signal-to-noise ratio. This significantly increases the allowable error-free data rate from 85 to 98.9 Gbit/s in the BtB case and from 81.2 Gbit/s to 95.3 Gbit/s in the 100 m OM5-MMF case.

Polarization-pinning in substrate emission multi-mode vertical-cavity surface-emitting lasers using deep trenches

We investigated the stable polarization-pinning properties of substrate emission InGaAs-based 980 nm multi-mode vertical-cavity surface-emitting lasers (VCSELs). For the multi-mode 40 um diameter aperture VCSELs, we introduced 30 μm wide, 9 μm depth deep trenches that are 15 μm away from the cavity aperture. The VCSELs with trench structure produced higher transverse-electric (TE) polarized light output power, as compared with transverse-magnetic (TM) polarized light output power, namely, the effective TM polarization suppression was realized. These trench-etched VCSELs exhibited a 7.5 dB orthogonal polarization suppression ratio with 16.8 mW of light output power at 60 mA of current injection. The dominant TE polarization distribution was observed in polarization-resolved near-field images of spontaneous and stimulated emission due to the induced strain by the etched trenches.

Effect of Transmission-Line Contact Length on the 50-Gbit/s Data Encoding Performance of a Multimode VCSEL

Directly modulated 850-nm multimode vertical-cavity surface-emitting lasers (MM-VCSELs) with different oxide apertures and transmission microstrip lengths are compared on the transmission performance of the non-return-to-zero on-off keying (NRZ-OOK) and four-level pulse amplitude modulation (PAM-4) data formats. In this work, intrinsic and extrinsic responses of the MM-VCSEL are also discussed concurrently. By tuning the length of the transmission microstrip in VCSEL, the low reflection coefficient and the enhanced 3-dB modulation bandwidth are achieved. The inductance of the transmission microstrip in the series connection with the capacitance in the active region is optimized to reduce the power loss induced by imaginary impedance. The different oxide aperture sizes for MM-VCSEL are also studied to control the capacitance and photon density. More importantly, the 3-dB modulation bandwidth, impedance matching, slope efficiency, relative intensity noise (RIN), and mode partition noise (MPN) for the MM-VCSEL with various designs are discussed to determine the best device with the high-speed transmission capability. The optimal MM-VCSEL with a diameter of 7 µm oxide aperture and a length of 25 µm transmission microstrip successfully demonstrates 50-Gbit/s OOK and 84-Gbit/s PAM4 after using the pre-emphasis technique for future data-center applications.

Regional Pulsewidth and Delay Nonuniformity of Modulated 940 nm Vertical‐Cavity Surface‐Emitting Laser Array

Under pulsed modulation, the emission uniformity in terms of pulsewidth and delay of a 940 nm multimode vertical‐cavity surface‐emitting laser (MM‐VCSEL) array is characterized by analyzing the position‐dependent pulsating and chirping dynamics among localized 600 VCSEL elements. In addition, the MM‐VCSEL array biased nearly threshold current (Ith) of 350 mA with a low resistance of 0.72 Ω reveals serious impedance mismatching with a high reflection coefficient (Γ) of −0.97. The MM‐VCSEL array biased below threshold (at Idc: ≈0.9 Ith) reveals low‐impedance and high‐capacitance behavior to result in large modulation reflection with allowable −3 dB bandwidth of 0.8 GHz. The nonuniform optical intensity and delay distribution among all MM‐VCSEL elements corroborate that the gradually enlarged inductance attenuates the modulation current away from bond wires. By modulating the 0.9Ith‐biased MM‐VCSEL array with an 127 ps‐wide electrical pulse with a peak amplitude of 15.3 V, a multimode fiber probe collects the single MM‐VCSEL output with average power, peak voltage, and pulsewidth of 0.08 mW, 25.1 mV, and 602 ps. The delay time as large as 273.4 ps between the nearest and farthest parts of local MM‐VCSEL elements in the array determines its impulse response and uniformity for applications.

Effect of Chirped Dispersion and Modal Partition Noise on Multimode VCSEL Encoded With NRZ-OOK and PAM-4 Formats

Chirp-dispersion and data transmission analyses of the multimode vertical-cavity surface-emitting laser (MM-VCSEL) with a 7.5-μm oxide-confined aperture and a root-mean-square spectral linewidth of 1 nm is performed for intra-data-center application. For directly modulating the non-return-to-zero on-off keying (NRZ-OOK) and the 4-level pulse amplitude modulation (PAM-4) data streams, an optimized bias current of 17 mA and a data amplitude of 800 mV are selected to provide the MM-VCSEL with an output optical power of 6.1 mW and a transverse mode number of 17. Such an optimized operation enables the MM-VCSEL with a differential resistance of 45.3 Ω to approach the impedance matching by driving the circuit such that the -3-dB bandwidth can be achieved to 23.8 GHz. The relative intensity noise and mode-partition-noise of the MM-VCSEL are analyzed to perform the optimized NRZ-OOK/PAM4 data encoding. By utilizing the pre-emphasis technique to compensate for the intensity and phase variation, the encoded NRZ-OOK and PAM-4 can be respectively promoted to 61 Gbit/s and 96 Gbit/s in the back-to-back (BtB) case. After passing through the 100-m OM5 multi-mode-fiber (MMF) link to suffer the severe modal dispersion, the data rate would respectively degrade to 45 Gbit/s (or 4.5 Gbit·km/s) and 70 Gbit/s (or 7 Gbit·km/s) for OOK and PAM-4 transmission.

800-MHz Bandwidth Signal Transmission with Radio over Multi-Mode-Fiber for Cascaded IFoF-Based C-RAN Mobile Fronthaul

Millimeter-wave band signals have the advantage of a high capacity and have been used in fifth-generation (5G) mobile services. An analog radio over multi-mode fiber (A-RoMMF) employing a directly modulated vertical cavity surface emitting laser is a promising technology for relaying millimeter-wave band signals to its dead area because of the advantages of low latency, low cost, and low power consumption. One of the applications of A-RoMMF is a cascaded intermediate-frequency-over-fiber (IFoF)-based centralized radio access network (C-RAN) mobile fronthaul (MFH) system. We conducted two studies on A-RoMMF to extend the transmission distance and bandwidth to adapt it to the system. First, we evaluated the effect of employing a single-mode vertical cavity surface emitting laser (VCSEL) and a multi-mode VCSEL on the transmission characteristics of A-RoMMF. Next, we evaluated the transmission characteristics of A-RoMMF using a cost-efficient bias-tee consisting of an electrical circuit pattern, the frequency response of which has a slope inverse to that of A-RoMMF at the 28-GHz band, to suppress the channel power difference of two component carrier signals for a wide-bandwidth signal transmission. We conducted these evaluations using A-RoMMF itself and herein demonstrate its adaption to a cascaded IFoF-based C-RAN MFH system at the 28-GHz band. When employing it with a 200-m long multi-mode fiber, the system achieves 400-MHz/CC and two component carrier signal transmissions.

Review of VCSELs for Complex Data-Format Transmission Beyond 100-Gbit/s

Persistent efforts on developing advanced and complex data algorithms to enable the vertical-cavity surface-emitting lasers (VCSELs) for data beyond 100-Gbit/s has been reviewed in this paper. This endeavor not only elevates the bit rate of the IEEE802.3 standards for data center applications but also to initiate the unification of data format for wired and wireless network coverage. To date, the highest allowable data rates of the single-VCSEL-based optical link are 80 Gbit/s for the NRZ-OOK format under back-to-back (BtB) transmission 168 Gbit/s for the PAM-4 format over 150-m OM5 MMF, and the 224 Gbit/s for the QAM-OFDM format at BtB case. For the high-level complex data-format transmission with rigorous demand on the signal-to-noise ratio (SNR), this work demonstrates the most up-to-date 32-ray quadrature amplitude modulation generalized frequency division multiplexing (32-QAM GFDM) data algorithm with optimized amplitude and K value for encoding the multimode (MM) VCSEL. To achieve the higher spectral-usage efficiency based on the SNR spectrum for maximal transmission data rate, the bit-loading discrete multi-tone (DMT) technique is implemented by rearranging each subcarrier to the appropriate QAM levels. By utilizing 32-QAM GFDM and bit-loading DMT, the maximal data rate in the BtB case could respectively achieve 119.5 Gbit/s and 130 Gbit/s when operating the MM VCSEL at 55 °C. The transmission capacity of the VCSEL operated at 55 °C would decrease to 93.8 Gbit/s for 32-QAM GFDM and 83 Gbit/s for bit-loading DMT after propagating through OM5 MMF with modal dispersion.

Linearisation Method of DML-based Transmitters for Optical Communications Part II: Experimental Demonstration and Implementation Methods

In this paper, we report the first experimental demonstration of the Stretched A linearisation method. The theoretical framework of the method and related simulation studies have been presented in Part I of this work. Here, we apply the method on an 850 nm multimode vertical-cavity surface-emitting laser (VCSEL) specified for operation at 10 Gb/s and demonstrate good quality optical output waveforms with little non-linear distortion at 16 Gb/s using non-return-to-zero (NRZ) modulation. An experimental sensitivity analysis on the generation of the required modulating current components demonstrates large tolerance to parameter mismatch. In addition, it is shown that an adaptive gain scheme can improve the tolerance to the timing offset between the current components. Potential hardware implementations using either analog or digital electronics are also discussed.

Development and Characterization of 100 Gb/s Data Communication VCSELs

Multi-mode vertical-cavity surface-emitting lasers (VCSELs) with improved bandwidth and noise performance have been developed for 100 Gb/s data communication links over multi-mode fiber. The VCSELs are designed for the peak multi-mode fiber bandwidth at 850 nm to maximize reach. The small-signal modulation 3dB bandwidth exceeds 25 GHz over temperature and relative intensity noise is below -145 dB/Hz. Transmission of 53.125 GBd PAM-4 signals over a 100m OM4 fiber is demonstrated by using equalization on both transmit and receive side. The transmitter and dispersion eye closure quaternary (TDECQ) penalty is below 4 dB for VCSEL operating up to 75°C.

Polarization control and mode optimization of 850\xa0nm multi-mode VCSELs using surface grating

To control the polarization of 850 nm multi-mode oxide-confined vertical-cavity surface-emitting lasers (VCSELs), monolithically integrated surface gratings are investigated. VCSELs with different grating parameters are simulated by the rigorous coupled-wave analysis (RCWA) method. Optical field intensity of two polarization orientations with different grating periods and duty cycles is obtained. The mode characteristics of VCSELs with different grating parameters are achieved by FDTD method. 850 nm surface grating VCSELs are fabricated and characterized, a maximum orthogonal polarization suppression ratio (OPSR) of 20.7 dB is observed. Meanwhile, VCSEL’s output mode characteristics are improved by integrating the surface grating. Application of integrated surface grating technique to achieve dynamically polarization-stable multi-mode VCSELs which enable polarization division multiplexed data transmission, the data throughput over a given link can be doubled.

Optical Broadcasting Employing Incoherent and Low-Coherence Spatial Modes for Bi-Directional Optical Wireless Communications

We introduce recently developed space division multiplexing (SDM)-associated technologies into optical wireless communications (OWC) and propose to use incoherent and low-coherence spatial modes to realize optical broadcasting with reliable performance by mitigating coherent interference. We experimentally demonstrate reconfigurable optical beam steering using a programmable multi-plane light conversion (MPLC)-based mode multiplexer (MMUX) and realize point-to-multi-point optical wireless communications with low-loss up-link optical beam combining and reach extension using multimode fiber (MMF). Both incoherent and low-coherence spatial modes can be generated employing cost-efficient components such as a multimode vertical cavity surface emitting laser (MM-VCSEL) and broadband amplified spontaneous emission (ASE) noise source.

Prototyping of a 25 Gbps optical transmitter for applications in high-energy physics experiments

Development of optical links with 850 nm multi-mode vertical-cavity surface-emitting lasers (VCSELs) has advanced to 25 Gbps in speed. For applications in high-energy experiments, the transceivers are required to be tolerant in radiation and particle fields. We report on prototyping of a miniature transmitter named MTx+, which is developed for high speed transmission with the dual-channel laser driver LOCld65 and 850 nm VCSELs packaged in TOSA format. The LOCld65 is fabricated in the TSMC 65 nm process and is packaged in the QFN-40 for assembly. The MTx+ modules and test kits were first made with PCB and components qualified for 10 Gbps applications, and were tested for achieving 14 Gbps. The data transfer rate of the MTx+ module is investigated further for the speed of up to 25 Gbps. The LOCld65 is examined with post-layout simulation and the module design upgraded with components including the TOSA qualified for 25 Gbps applications. The PCB material is replaced by the Panasonic MEGTRON6. The revised MTx+ is tested at 25 Gbps and the eye-diagram shows a mask margin of 22%.

Nano-height cylindrical waveguide in GaN-based vertical-cavity surface-emitting lasers

We have proposed a novel nano-height cylindrical waveguide in blue GaN-based vertical-cavity surface-emitting lasers (VCSELs). The proposed 5 nm step height cylindrical waveguide using simple processes provides lateral optical confinement and excellent performance in terms of light output power (LOP) and wall plug efficiency (WPE). A fabricated multi-mode VCSEL with 7 μm emission diameter achieved a LOP of 23.7 mW, maximum external differential quantum efficiency of 43.6%, and WPE of 10% under continuous wave operation at 20 °C. Additionally, a single-mode VCSEL with 3.3 μm emission diameter exhibited LOP of more than 5 mW and WPE of 9.9%.

Si3N4 photonic integration platform at 1 µm for optical interconnects.

Vertical-cavity surface-emitting lasers (VCSELs) are the predominant technology for high-speed short-range interconnects in data centers. Most short-range interconnects rely on GaAs-based multi-mode VCSELs and multi-mode fiber links operating at 850 nm. Recently, GaAs-based high-speed single-mode VCSELs at wavelengths > 1 µm have been demonstrated, which increases the interconnect reach using a single-mode fiber while maintaining low energy dissipation. If a suitable platform for passive wavelength- and space-multiplexing were developed in this wavelength range, this single-mode technology could deliver the multi-Tb/s interconnect capacity that will be required in future data centers. In this work, we show the first passive Si3N4 platform in the 1-µm band (1030-1075 nm) with an equivalent loss < 0.3 dB/cm, which is compatible with the system requirements of high-capacity interconnects. The waveguide structure is optimized to achieve simultaneously single-mode operation and low bending radius, and we demonstrate a wide range of high-performance building blocks, including arrayed waveguide gratings, Mach-Zehnder interferometers, splitters and low-loss fiber interfaces. This technology could be instrumental in scaling up the capacity and reducing the footprint of VCSEL-based optical interconnects and, thanks to the broad transparency in the near-infrared and compatibility with the Yb fiber amplifier window, enabling new applications in other domains as optical microscopy and nonlinear optics.

Experimental Study of Multistability in a Multimode Vertical-Cavity Laser

The coexistence of various spatial distributions of laser intensity in a multimode vertical-cavity surface-emitting laser (VCSEL) at fixed DC values was experimentally demonstrated. Their structures were characterized by different integrated intensities, polarization states, and sets of wavelengths. Spatial multistability was shown to lead to the appearance of multistability in the laser intensity and polarization multistability and was characterized by a strong local dependence of the laser intensity hysteresis behavior on the pump current. The effect of the diode-laser temperature on the manifestation of multistability was also investigated.

Equivalent Circuit Model of High-Performance VCSELs

In this work, a general equivalent circuit model based on the carrier reservoir splitting approach in high-performance multi-mode vertical-cavity surface-emitting lasers (VCSELs) is presented. This model accurately describes the intrinsic dynamic behavior of these VCSELs for the case where the lasing modes do not share a common carrier reservoir. Moreover, this circuit model is derived from advanced multi-mode rate equations that take into account the effect of spatial hole-burning, gain compression, and inhomogeneity in the carrier distribution between the lasing mode ensembles. The validity of the model is confirmed through simulation of the intrinsic modulation response of these lasers.

Real-time 850 nm multimode VCSEL to 1 550 nm single mode VCSEL data routing for optical interconnects

Short-reach optical interconnects among massive serves in data centers have attracted extensive research recently. Increasing capacity, cost and power efficiency as well as wavelength switching between data center network nodes are still key challenges for current optical interconnects. In this work, we experimentally demonstrate the real-time inter-mode optical wavelength switching technique, for high-speed wavelength flexible data center interconnects. A 10 Gbit/s 1 550 nm single mode vertical cavity surface emitting laser (VCSEL) is optically injected and used to control a 10 Gbit/s multimode VCSEL carrier at 850 nm. Results show that a clearly open eye diagram is achieved at back-to-back analysis, implying a successful wavelength switch and error-free operation at 10 Gbit/s. A fully optical wavelength conversion of a multimode VCSEL operation at 850 nm using a single mode VCSEL subject to external optical injection at 1 550 nm is reported. This work opens new perspectives towards the development of a cost effective high-speed real-time inter-band wavelength switching technique between servers and network devices operating at different transmission windows at network nodes, for current and future optical interconnects.

VCSEL-based broadband wavelength converter for ultra-wideband flexible spectrum data routing in optical interconnects

Abstract High-speed real-time wavelength conversion at key data centre network nodes is an emerging fundamental functionality requirement for transparent content resolution, wavelength assignment and routing towards better utilization of network resources under dynamic traffic patterns. In this paper, we experimentally demonstrate the first vertical cavity surface emitting laser (VCSEL)-based broadband wavelength converter for ultra-wide wavelength routing in Datacom. An 850 nm multimode VCSEL is directly modulated with 8.5 Gbps data and successfully transmitted error-free over 100 m OM3 multimode fibre (MMF) with a receiver sensitivity of −16.04 dBm, and a total transmission penalty of 1.14 dB. The received data is then used to run two cascaded single mode VCSELs at 1550 nm within the network integration node, therefore achieving the first reported real-time inter-band wavelength converter to C-band. By exploiting VCSEL wavelength tuneability with changing bias current, we show that the converted wavelengths can be routed over 3.2 nm spectra range for integration into ITU-T 50 GHz flexible grid and beyond. The newly converted wavelengths are successfully transmitted over 24.7 km of G 655 single mode fibre, achieving a minimum receiver sensitivity of −14.28 dBm, with a maximum transmission penalty of 2.72 dB. This work offers a viable enabling development technology for broadband wavelength converters for real-time wavelength routing in optical interconnects to address content resolution and wavelength assignment problem for current and future Datacom.

Time and reference frequency transfer over 850 nm VCSEL for digital telecommunication network synchronization

Accurate time transfer and synchronization between different network nodes is a key functional requirement in digital communication. Developments in optical fiber-based frequency dissemination techniques have improved optical frequency stability over time to much lower levels. In this work, we experimentally present the reference frequency transfer employing 850 nm vertical cavity surface emitting laser (VCSEL) over 100.3 m OM3 multimode fiber for synchronization of clocks on networked devices such as servers and racks/pod at different data center network nodes. A low-cost power-efficient multimode VCSEL with central wavelength at 844.26 nm is directly modulated with a 2 GHz reference frequency (RF) clock signal, and transferred over 100.3 m of OM3 multimode fiber. The single side band (SSB) phase noise of -104.62 dBc/Hz and −100.70 dBc/Hz is experimentally measured at back-to-back (B2B) and 100.3 m OM3 multimode fiber transmission respectively at a 1 kHz frequency offset. The jitter stability of 0.14 ps and 0.15 ps is experimentally achieved at B2B and 100.3 m fiber transmission, respectively. This work provides an alternative viable approach for the development of time keeping devices in high-speed short-reach optical communication systems.