Vertical-cavity Surface-emitting Laser Module Research Articles

Anomalous noise spikes beating from a fine-splitting mode of surface-emitting laser with imperfect true-roundness aspect

High-frequency discrete and distinct mode-beating noise (MBN) spectra are an anomalous phenomenon observed in the related intensity noise spectrum from the vertical-cavity surface-emitting laser (VCSEL), which reveals strong correlation with fine-splitting transverse modes caused by non-circular emission aspect of the VCSEL with imperfect true-roundness. Such a phenomenon is demonstrated for the first time, which seriously degrades the transmission performance of high-speed and broad-band encoding data carried by the multi-mode (MM) VCSEL biased under a large DC bias. In this work, the 4-ary quadrature-amplitude modulation (4-QAM) orthogonal frequency-division multiplexing (OFDM) and the non-return-to-zero on–off keying (NRZ-OOK) transmissions with the direct modulation of different 850-nm MM VCSELs, as suffered from the MBN-induced notches of the signal-to-noise-ratio (SNR) spectra, are characterized under their high-level lasing conditions with multi-mode numbers. The significant SNR degradation on the receiving and decoding performances caused by the cross-correlation between MBN and data spectra is also examined by tuning the lensed fiber along the transverse (X/Y)-axis of the VCSEL (perpendicular to the surface normal) when collimating the output beam, which finds the disorderly shifted MBN spectra, as attributed to the collection of light emitted from the overlapped gain region of different transverse modes. In contrast, the MBN reveals invariant spectral features while the lensed fiber is either back-and-forth tuning along the vertical (Z)-axis (parallel with the surface normal) or tilting its angles from the surface normal under the same horizontal position and distance concerning the MM VCSEL. Such MM VCSEL exhibits several MBN spikes to decay the SNRs of NRZ-OOK and QAM OFDM data encoded around 21 and 34 GHz. For other MM VCSELs with reducing numbers of their transverse lasing modes and deforming roundness of their circular emitting aspects, the corresponding MBN spectral peaks can also be respectively observed at 14/17/24 GHz and 4.5/10.5/13 GHz to degrade the SNRs of the encoded data at such frequencies. Even though pre-emphasized encoding technology is employed, such an MBN effect is strongly correlated with the asymmetric circular aspect of the VCSEL with a degraded roundness ratio, which still affects the data modulation performance of the MM VCSEL for data center communication.

Development of high-power VCSEL emitter of size 30mm x 30mm, scalable in two dimensions and applicable to Si wafer annealing

Two-dimensionally scalable vertical cavity surface emitting laser (VCSEL) emitter of size 30mm×30mm was designed and fabricated for practical applications in the rapid heating processes of large-area targets. Nine emitters were arranged in a 3×3 array for an experimental VCSEL module, which was used for annealing of a 4” Si wafer. Each emitter was composed of 120 chips of size 1.26mm×1.26mm, and each chip was composed of 770 VCSEL diodes of 30μm in diameter, emitting a continuous beam with wavelength of 940nm and maximum optical power density of 47.6W/cm2. Since VCSELs generated a significant amount of Joule heat during operation, an effective cooling system was required for reliable thermal management of the emitter. A water cooling block of 126 microchannel flow passages with the heat transfer area-to-volume ratio approximately 1270m2/m3 was designed and fabricated for each emitter, resulting in the thermal resistance of 0.012K/W, equivalent to the overall heat transfer coefficient of 92,000W/m2K at water flow rate of 1L/min, which was the minimum value for the normal operation of the emitter. By comparing the experimental images on the 4” Si wafer irradiated by the single emitter and by the module with the numerical predictions based on the ray-tracing method, the laser beam emitted from a single VCSEL diode of 30μm in diameter was estimated to have the mode of TEM01* (doughnut profile, multimode) and a divergence angle of 20∘. These were two factors that affected the irradiation uniformity on the target. In addition, to validate the applicability of the high-power VCSEL system to anneal large-area targets, the heating process of 4” Si wafers using the module of nine emitters was experimentally carried out at atmospheric pressure, and was compared with numerical predictions.

Low-Cost Interrogation of Long-Distance and Multipoint FBG Sensor Using Incoherent-FMCW Optical Ranging System

More than a decade, multipoint sensor systems involving fiber Bragg gratings (FBGs) have been widely used in various implementations, especially the structural health monitoring system (SHMS). In this paper, a three-points FBG sensor was successfully analyzed by combination of incoherent frequency modulated continuous wave (I-FMCW) optical ranging system and a vertical-cavity surface-emitting laser (VCSEL) as a wavelength-tunable laser source. This system is not only capable of reading out the FBG sensors, but also identifying the location of the installed FBGs. The I-FMCW can be conducted by intensity modulation of a low-cost commercial VCSEL. Furthermore, the wavelength of the VCSEL can also be tuned by the injection current change to match the Bragg wavelength of FBGs. The detected signal from the FBGs and the reference signal from the signal generator are electrically mixed to obtain the beat frequency. A thermo-electric controller (TEC) controls temperature of one of the FBGs in the range of 25° C ~ 45° C, and contrary the others are kept at room temperature. As well as measuring temperature, one of the FBGs is strained in the range of $369~\mu \varepsilon \sim ~2137~\mu \varepsilon $ . The measurement result shows that the system can clearly distinguish all FBGs installed in totally 6.6 km-long optical fiber. In addition, the Bragg wavelength shift of the FBGs according to temperature as well as strain changes was successfully measured. The system can be scaled up more than three FBGs and extended scale area, and is a candidate as a low-cost and simple FBG interrogator for a long-range SHMS.

Enabling Equalization and Soft Decision by k-Means for VCSEL-Based PAM-4 Optical Interconnection

As for vertical-cavity surface-emitting lasers (VCSEL)-based optical pulse amplitude modulation (PAM) interconnection system, apart from the universal signal distortions like Gaussian noise and fiber dispersion, direct modulation of VCSEL specially induces level nonlinearity and level-dependent noise. To mitigate these two specific distortions in VCSEL-based link, a k-means assisted PAM-4 signal receiver including equalization and soft decision (SD) is proposed. Based on learned mean values and standard deviations for individual levels through k-means approach, probability density function of nonlinearly distorted PAM-4 symbols can thus be approximated more precisely than conventional SD, leading to improved decision performance. In addition, depending on k-means result, least means square (LMS) equalization with adapted levels can converge to a lower error value than conventional LMS, thanks to level nonlinearity mitigation. Consequently, by using proposed k-means equalization and SD, improved signaling performance can be obtained by mitigating the level nonlinearity and level-dependent noise. Both simulation and experiment are performed for evaluating the proposed method in a 100-Gb/s VCSEL-MMF PAM-4 optical interconnection system, by comparing with conventional means.

Applications of vertical cavity surface emitting lasers for low-pressure chemical vapor deposition reactors

Vertical cavity surface emitting lasers (VCSELs) with a wavelength of 980 nm were applied to design a low-pressure chemical vapor deposition (LPCVD) reactor as a promising heat source of excellent irradiation uniformity, rapid power controllability, and extended spatial scalability.The average divergence angle of the laser beam emitted from a myriad of cells was estimated from a comparison of the VCSEL emission distribution obtained from ray-tracing calculation with experimental measurements using a power meter. This property was closely related to irradiation uniformity and spatial scalability.The experimental temperature distribution of silicon wafers exposed to high power VCSEL beams was used to verify the performance of commercial codes on three-dimensional simulation models including conduction, convection and thermal radiation. For LPCVD reactor design, this code was used to predict the optimal placement of the VCSEL module for uniform irradiation on 300 mm diameter wafers.The best-fit values of the factors for the Arrhenius equation describing the deposition process of polycrystalline silicon (poly-Si) thin-film using silane gas species were obtained from a comparison of deposition rates from numerical simulations with experimental observations available in the literature. Using these factors and the optical characteristics of the VCSEL and Si, the deposition process on the wafer with 300 mm in diameter was simulated under various operation conditions. Especially, instead of the uniform surface temperature condition, the heat flux distribution estimated from experimental irradiations on the wafer was used for the boundary condition at the bottom surface of the wafer in order to realize a practical deposition process.Simulation using the realistic heat flux boundary conditions showed that deposition rates in the wafer edge-region were significantly reduced due to the severe temperature drop in that region, compared to that using the ideal uniform temperature condition. In order to minimize this wafer exclusion area, the emission power of the VCSEL emitters affecting the wafer edge-region was adjusted to slightly increase, resulting in a more uniform distribution of the deposition rate.

High-Speed VCSEL-Based Transceiver for 200 GbE Short-Reach Intra-Datacenter Optical Interconnects

The soaring demand for higher speeds in datacenters to address the relentless growth of the global IP traffic places optical interconnects in the spotlight. In this manuscript, we present a high-speed optical transceiver for intra-datacenter connectivity. The transceiver is based on single-mode, single-polarization high-speed vertical-cavity surface-emitting lasers (VCSELs), a VCSEL driver chip, and a linear receiver. Following a step-by-step approach, we present the architectures, assembly processes, and experimental results from the different modules. More specifically, we demonstrate (1) a data transmission experiment at 80 Gb/s using PAM-4 (four-level Pulse Amplitude Modulation) modulation for a reach of up to 500 m by employing a single-mode VCSEL module, and (2) a full-link experiment proving up to 64 Gb/s per lane capacity using PAM-4 signaling of the VCSEL-based optical transceiver test vehicles in back-to-back configuration and up to 56 Gb/s for 500 m and 2 km transmission distances. The acquired experimental results verify the suitability of the optical transceiver for intra-datacenter interconnects’ applications.

Chirp Evaluation of 850-nm Single Mode VCSEL Exploiting Modal Noise in Standard Single Mode Fiber

The phenomenon of modal noise, which is typically undesired within fiber optic links, is exploited advantageously to characterize vertical cavity surface emitting lasers (VCSELs) operating in the first optical window (wavelength λ ≃ 850 nm), in terms of frequency chirping. The proposed method, which can be applied utilizing ordinary laboratory components and instrumentation, contributes to fill an existing gap. Indeed, the methods which are typically exploited to determine this undesired optical frequency modulation can be straightforwardly applied only to sources operating at wavelengths of 1310 and 1550 nm, while they become unpractical or even inapplicable at wavelengths of 850 nm. Through the proposed technique, the evaluation of chirp coefficient M and phase difference ψ between frequency and intensity modulation (or equivalently linewidth enhancement factor α and chirp scaling factor κ) of VCSELs can be performed. This allows the correct design of various types of fiber optic links, making possible to take advantage of these low-cost and low-consumption optical sources.

Impact of Damping on 50 Gbps 4-PAM Modulation of 25G Class VCSELs

We present an investigation into the effects of photon lifetime and the associated damping of the modulation response on 50 Gbps 4-pulse-amplitude modulation (4-PAM) signal generation with directly modulated 25G class vertical-cavity surface-emitting lasers (VCSELs). While 4-PAM benefits from higher slope efficiency and output power, it is shown that the greater impact of intensity noise and dynamic nonlinearities implies that a VCSEL with a longer photon lifetime and more damped response is needed when transitioning from existing 25 Gbps OOK to 50 Gbps 4-PAM using the 25G VCSEL technology.

Chirp-based direct phase modulation of VCSELs for cost-effective transceivers.

A 2.5 Gb/s differential binary phase-shift keying (DPSK) transmitter based on direct phase modulation of a vertical cavity surface emitting lasers (VCSEL) using its own chirp is proposed. The VCSEL, which has a wavelength of 1539.84 nm, has been characterized both statically and dynamically. The sensitivity of a single photodiode heterodyne receiver using the proposed 2.5 Gb/s VCSEL transmitter is -39.5 dBm. Thus, this transmitter is an extremely cost-effective solution for future access networks.

High Speed VCSEL Transmission at 1310 nm and 1550 nm Transmission Wavelengths

Vertical cavity surface emitting lasers (VCSELs) operating at 1310 nm and 1550 nm are very promising sources for access and interconnections in telecommunication systems due to their technologically attractive properties such as low threshold currents and narrow spectral linewidth due to single mode operation. VCSELs are also being rapidly commercialized for single-mode fibre metropolitan area and wide area network applications. All these advantages leads to cost-effective wavelength-tunable lasers, which are essential for the future intelligent, all-optical networks. Direct modulation (DM) of VCSEL with separate optical and current apertures enables high modulation bandwidth operating at single mode at low current density. However, dispersion and attenuation is a major hurdle to VCSELs transmission at bit rate of 10 Gb/s and above. In this study, a 1310 and 1550 nm VCSELs were directly modulated with 10 Gb/s NRZ PRBS 2⁷-1 and transmitted over 25 km ITU. T G.652 and ITU. T G.655 fibres and optimized for metro-access distances. A low dispersion penalty was realized when a 1550 nm VCSEL was used on a G.655 fibre and when a 1310 nm source was transmitted over a G.652 fibre. The transmission system has been analyzed on the basis of different parameters, which are (Bit error rate) BER, Quality (Q) factor and Output power.

Modeling of noise and distortion associated with ultra‐high‐speed modulation of VCSEL with slow‐light feedback

AbstractThis paper presents numerical modeling on the noise properties and signal distortion associated with millimeter‐frequency modulation of vertical‐cavity surface‐emitting laser (VCSEL) under with a transverse‐coupled cavity. The study is based on a time‐delay rate equation model that takes into account the multiple round trips in the feedback cavity and the optical loss and phase delay in each round trip. Strong slow‐light feedback is found to boost the modulation bandwidth to frequencies approaching 70 GHz and induce resonance modulation due to photon–photon resonance (PPR) over passbands centered on frequencies reaching 90 GHz. We show that the relative intensity noise of the VCSEL with resonance modulation is enhanced when the noise frequency approaches the corresponding PPR frequency VCSEL. The same effect applies for the VCSEL with extended carrier‐photon resonance (CPR) at the CPR frequency. The low‐frequency part is characterized by flat (white) noise of level nearly equal to −140 dB/Hz. The second‐harmonic distortion (2HD) values are smaller than −10 dB under small‐signal modulation and increase to lower than −5 dB when the modulation index becomes 0.3. Copyright © 2015 John Wiley & Sons, Ltd.

Enhancing the modulation bandwidth of VCSELs to the millimeter-waveband using strong transverse slow-light feedback.

We present modeling on the millimeter (mm)-wave modulation of vertical-cavity surface-emitting laser (VCSEL) with a transverse coupled cavity (TCC). We show that strong slow-light feedback can induce 300% boosting of the modulation bandwidth of the TCC-VCSEL. Also, the strong lateral feedback can induce resonance modulation over passbands centered on frequencies as high as 3.8 times the VCSEL bandwidth. The slow-light feedback is modeled by a time-delay rate equation model that takes into account the multiple round trips as well as the optical loss and phase delay in each round trip in the feedback cavity.

Noise performance of high-speed radio over fiber links employing vertical-cavity surface-emitting lasers

This study investigates the intensity noise in high-speed vertical-cavity surface-emitting lasers (VCSELs) and its contribution to the noise performance of radio over fiber (RoF) links. We evaluate the sinusoidal modulation of VCSELs in terms of the second-order harmonic distortion (2HD) and third-order intermodulation distortion (IMD3) in additions to the relative intensity noise (RIN). The spurious-free dynamic range of the proposed VCSEL is estimated. The noise performance of the RoF link is assessed by the noise figure. The modulation characteristics of the VCSEL and the gain and noise factor (NF) of the fiber link are compared under conventional and high-speed modulations of VCSELs. Also, we present comparison of the NF between short (300 m) and relatively long (2 km) fibers.

Advances and new functions of VCSEL photonics

A vertical cavity surface emitting laser (VCSEL) was born in Japan. The 37 years’ research and developments opened up various applications including datacom, sensors, optical interconnects, spectroscopy, optical storages, printers, laser displays, laser radar, atomic clock and high power sources. A lot of unique features have been already proven, such as low power consumption, a wafer level testing and so on. The market of VCSELs has been growing up rapidly and they are now key devices in local area networks based on multi-mode optical fibers. Optical interconnections in data centers and supercomputers are attracting much interest. In this paper, the advances on VCSEL photonics will be reviewed. We present the high-speed modulation of VCSELs based on a coupled cavity structure. For further increase in transmission capacity per fiber, the wavelength engineering of VCSEL arrays is discussed, which includes the wavelength stabilization and wavelength tuning based on a micro-machined cantilever structure. We also address a lateral integration platform and new functions, including high-resolution beam scanner, vortex beam creation and large-port free space wavelength selective switch with a Bragg reflector waveguide.

High-peak-power vertical-cavity surface-emitting laser quasi-array realized using optimized large-aperture single emitters

We report on a high-peak-power vertical-cavity surface-emitting laser (VCSEL) quasi-array emitting at approximately 980 nm. The quasi-array is composed of four tightly packaged single emitters connected in series. The external quantum efficiency of large-aperture VCSEL was improved by optimizing the current distribution, and single emitters with a 62 W peak power were developed as the basic building block of this quasi-array. More than 210 W peak power is demonstrated from a ∼2.2 × 2.2 mm2 quasi-array at a current of 110 A under 30 ns and 5 kHz pulsed operation. This is the first report on a hundreds-of-watts VCSEL module consisting of single emitters.

Modulation Characteristics of High-Speed and High-Temperature Stable 980 nm Range VCSELs Operating Error Free at 25 Gbit/s up to 85 °C

High-speed oxide-confined 980 nm vertical cavity surface emitting lasers (VCSELs), exhibiting very temperature stable static and dynamic characteristics, are presented, which are well-suited for future optical data link and high-performance computing applications. Error-free operation at data rates of 25 Gbit/s in a wide temperature range from 25 to 85 °C is achieved without the need to adjust any of the electrical driving parameters. Small-signal modulation of VCSELs with different oxide aperture diameters is analyzed to identify the primary mechanisms that control device modulation rate by extracting the standard two-pole rate equation parameters.

VCSEL Module With Polymer Optical Output Rods to Enable High Efficiency Coupling for Optical Interconnection

New vertical-cavity surface-emitting laser (VCSEL) modules-designed with a optical output rod surrounded by cladding-have been proposed to realize high-efficiency low-cost optical interconnection. Prototypes have been fabricated using a photomask transfer method employing two kinds of ultraviolet curable resin. Observation of the near-field pattern and eye pattern for signal transmission shows that output rods with a diameter of 50-mum efficiently confine the laser beam as an optical waveguide. In addition, ray tracing simulation indicates that this new VCSEL offers greater positional tolerance-as much as +18/ -22 mum-for coupling to optical wiring in 90deg light path conversion.

High-Speed Modulation of Index-Guided Implant-Confined Vertical-Cavity Surface-Emitting Lasers

An etched photonic crystal (PhC) or holey wedge structure induces index confinement into 850-nm implant-confined vertical-cavity surface-emitting lasers (VCSELs) to engineer the spatial overlap between the optical mode and laser gain for improved high-speed operation and reduced relative intensity noise. Large-signal operation of 12.5 Gb/s is achieved with a single transverse-mode PhC VCSEL and 15 Gb/s with a single transverse-mode holey VCSEL. An excessive current diffusion effect is found when the difference between the electrical and optical diameter is large (>4 mum), which limits the large-signal modulation of single-mode VCSELs. The design rules for optimal single transverse-mode high-speed PhC and holey VCSELs are extracted from a parametric study of their large-signal modulation characteristics.

Self-Alignment of Optical Devices With Fiber for Low-Cost Optical Interconnect Modules

We propose a novel self-alignment process of optical devices with optical fiber. A vertical-cavity surface-emitting laser (VCSEL) was automatically coupled with a multimode fiber (MMF) through the surface tension of a liquid adhesive within 1.5 s. Misalignment between the center of the VCSEL and the fiber was measured to be 15 mum, which is acceptable for coupling the VCSEL with the MMF. High-speed pulse modulation of the self-aligned VCSEL up to 5 Gb/s, as well as at 1 Gb/s, was demonstrated. The average optical output power was as high as -5.9 dBm at 1 Gb/s.

Direct high-frequency modulation of VCSELs and applications in fibre optic RF and microwave links

With the rapid development of wireless communication networks there is an increasing demand for efficient and cost-effective transmission and distribution of RF signals. Fibre optic RF links, employing directly modulated semiconductor lasers, provide many of the desired characteristics for such distribution systems and in the search for cost-effective solutions, the vertical cavity surface emitting laser (VCSEL) is of interest. It has therefore been the purpose of this work to investigate whether 850 nm VCSELs fulfil basic performance requirements for fibre optic RF links operating in the low-GHz range. The performance of single- and multimode oxide confined VCSELs has been compared, in order to pin-point limitations and to find the optimum design. Fibre optic RF links using VCSELs and multimode fibres have been assembled and evaluated with respect to performance characteristics of importance for wireless communication systems. We have found that optimized single-mode VCSELs provide the highest performance and that links using such VCSELs and high-bandwidth multimode fibres satisfy the requirements in a number of applications, including cellular systems for mobile communication and wireless local area networks.