850-nm Vertical-cavity Surface-emitting Lasers Research Articles

Very High Bit-Rate Distance Product Using High-Power Single-Mode 850-nm VCSEL With Discrete Multitone Modulation Formats Through OM4 Multimode Fiber

In order to investigate the tradeoff between optical spectral width and modulation speed of 850-nm Zn-diffusion vertical-cavity surface-emitting laser (VCSEL) and its influence on the performance of discrete multitone (DMT) modulation, two kinds of high-speed VCSEL structures with different cavity lengths (λ/2 and 3λ/2) are studied. By shortening the cavity length to λ/2, allocating the oxide layer in the standing-wave peak, and performing a Zn-diffusion aperture in our VCSEL structure, stable dual mode in the output optical spectra across the full range of bias currents with good high-speed performance (∼16-GHz bandwidth) can be achieved. Compared with its multimode reference, it shows far less roll-off with regard to the maximum data rate versus transmission distance over OM4 multimode fibers under forward error correction (FEC) threshold (BER $ ). On the other hand, for the 3λ/2 VCSEL structure, by using the same Zn-diffusion conditions as those of dual-mode counterpart, highly single-mode operation (side-mode suppression ratio> 35 dB) with high available power can be achieved over the full range of bias currents. Although such device shows a smaller 3-dB electrical-to-optical bandwidth (12 versus 16 GHz) than that of the dual-mode one, it exhibits a superior transmission performance by use of DMT modulation format. A record high bit-rate distance product (107.6 Gb/s·km) at nearly 50-Gb/s transmission under FEC threshold (BER $ ) through 2.2-km OM4 fibers has been successfully demonstrated by the use of single-mode VCSEL with optimized structures. In addition, error-free (BER $ ) transmission at 20.3 Gb/s with bit-rate distance product of 44.66 Gb/s·km has also been demonstrated.

High-Speed Inverted-Polarity Oxide-Confined Copper-Plated 850-nm Vertical-Cavity Lasers

Inverted-polarity ( n -up), high-speed, oxide-confined, polyimide-planarized, copper-plated 850-nm vertical-cavity surface-emitting lasers with various aperture sizes were fabricated and characterized. The reported devices demonstrated intrinsic, parasitic, and thermal maximum bandwidth limitations of 39.3, 24.6, and 22.9 GHz, respectively. VCSELs with 7 μm active area diameter and 4 μm of plated copper exhibited a maximum –3 dB frequency modulation bandwidth ( $f_{-3\;{\bf dB}\; {\rm max}}$ ) of 18.8 GHz and a resonance frequency of 14.8 GHz at a bias current density ( $J_{{\rm bias}}$ ) of only 8.8 kA/cm2, limited by thermal effects, with a high modulation current efficiency factor of 17.0 GHz/mA1/2 for quantum well VCSELs. The presented VCSELs also demonstrated a record high $f_{-3\;{\rm dB}\; {\rm max}}^{2}/J_{{\rm bias}}$ ratio of 40.2 GHz2/kA/cm2 which represents an 11% increase compared with the highest previously reported ratio. Rate-equation-based thermal VCSEL model and three-pole transfer function approximation were applied to extract several VCSELs’ parameters, which enabled the estimation of the VCSELs internal temperature and major bandwidth limitation. Particle swarm optimization was utilized in parameter extraction and optimization.

Measurement of Temperature-Dependent Polarization Parameters in Long-Wavelength VCSELs

Measurements of the polarization-resolved characteristics of a 1550-nm vertical-cavity surface-emitting laser (VCSEL) as a function of the temperature are presented. Type I (from the high-frequency to the low-frequency polarization mode) and Type II (from the low-frequency to the high-frequency polarization mode) polarization switchings (PS) are found in our device. Double PS (Type II followed by Type I and vice versa) are found when increasing the bias current for different device temperatures. Type II polarization switching to the gain disfavored mode is obtained. PS current, nonlinear dichroism, differential gain, and threshold current are measured as a function of the temperature. An expression relating these quantities to the spin-flip rate is derived. Using this expression, we have found large values of the spin-flip rate and its temperature dependence. The spin-flip rate increases with temperature except in a narrow temperature region in which a local minimum is observed. This behavior is mainly determined by the dependence of the inverse of the nonlinear dichroism on the temperature.

Two-Mode Coexistence in 1550-nm VCSELs With Optical Feedback

Two-mode coexistence in 1550-nm vertical-cavity surface-emitting lasers with optical feedback is experimentally investigated in detail. With the increase of the feedback strength, the two-mode coexistence region is enlarged for continually varied current. The feedback delay time has little influence on the two-mode coexistence region distribution under our experimental conditions. Moreover, a mapping of polarization-mode suppression ratio between two linear polarization (LP) modes is presented in the parameter space of the bias current and the feedback strength. In addition, the simultaneous chaos outputs of two LP modes are obtained for a certain two-mode coexistence region.

High-Speed VCSELs With Strong Confinement of Optical Fields and Carriers

We present the design, fabrication, and performance of our latest generation high-speed oxide-confined 850-nm vertical-cavity surface-emitting lasers. Excellent high-speed properties are obtained by strong confinement of optical fields and carriers. High-speed modulation is facilitated by using the shortest possible cavity length of one half wavelength and placing oxide apertures close to the active region to efficiently confine charge carriers. The resulting strong current confinement boosts internal quantum efficiency, leading to low threshold currents, high wall-plug efficiency, and state-of-the-art high-speed properties at low bias currents. The temperature dependent static and dynamic performance is analyzed by current-power-voltage and small-signal modulation measurements.

Temperature-Dependent Impedance Characteristics of Temperature-Stable High-Speed 980-nm VCSELs

The temperature dependence of the impedance characteristics of high bit rate, highly temperature-stable 980-nm oxide-aperture vertical-cavity surface-emitting lasers (VCSELs) is investigated. A small-signal equivalent circuit model is fitted to measured small-signal reflection S11 and S21 scattering parameters across a large range of operating bias currents, temperatures, and oxide-aperture diameters to extract the circuit elements, −3 dB modulation bandwidth, relaxation resonance frequency, and parasitic cutoff frequency for each particular operating condition. The parasitic cutoff frequency of our small oxide-aperture diameter VCSELs is highly temperature insensitive from room temperature up to 85 °C, and does not limit our VCSELs’ maximum data transmission rate. Finally, the dependence of the impedance characteristics and modulation bandwidth of our VCSELs on the oxide-aperture diameter is analyzed at 25 °C and 85 °C. The larger capacitance is the main reason of our larger aperture VCSELs, compared with smaller aperture VCSELs, have a lower parasitic cutoff frequency.

Polarization-Stable 980nm Vertical-Cavity Surface-Emitting Lasers with Diamond-Shaped Oxide Aperture**Supported by the National Natural Science Foundation of China under Grant Nos 61222501 and 61335004, and the Specialized Research Fund for the Doctoral Program of Higher Education of China under Grant No 20111103110019.

Polarization-stable 980nm oxide-confined vertical-cavity surface-emitting lasers with 3 μm diamond-shaped aperture are fabricated by comprehensively utilizing the anisotropic properties of wet etching and wet nitrogen oxidation of III–V semiconductor materials. Polarization-stable operation along the major axis of the diamond-shaped oxide aperture with 11 dB orthogonal polarization suppression ratio is achieved in a temperature range of 15–55°C from the threshold to 4 mA.

A 71-Gb/s NRZ Modulated 850-nm VCSEL-Based Optical Link

We report error free (BER <; 10-12) operation of a directly non-return-to-zero modulated 850-nm vertical cavity surface-emitting laser (VCSEL) link operating to 71 Gb/s. This is the highest error free modulation rate for a directly modulated laser of any type. The optical link consists of a 130-nm BiCMOS driver IC with two-tap feed-forward equalization, a wide bandwidth 850-nm VCSEL, a surface illuminated GaAs PIN photodiode, and a 130-nm BiCMOS receiver IC.

Impact of Damping on High-Speed Large Signal VCSEL Dynamics

An investigation of the optimal relaxation oscillation damping for high-speed 850-nm vertical-cavity surface-emitting laser (VCSEL) under large signal operation is presented, using devices with K -factors ranging from 0.1 to 0.4 ns. Time-domain measurements of turn-on transients are used to quantify damping dependent rise times, overshoots, and signal amplitudes. Optical eye diagrams together with timing jitter and bit error rate measurements reveal a tradeoff between the rise time and the duration of the relaxation oscillations. To produce a high-quality eye at a specific data rate, a proper amount of damping is needed to simultaneously obtain sufficiently high bandwidth and low timing jitter. We found that for error-free transmission, a VCSEL with a 0.3 ns K-factor achieved the best receiver sensitivity at 10 and 25 Gb/s, whereas a less damped VCSEL with a 0.2 ns K-factor achieved the best sensitivity at 40 Gb/s.

VCSEL Arrays for Multicore Fiber Interconnects With an Aggregate Capacity of 240 Gb/s

We report on the performance and uniformity of dense 2-D six-channel 850-nm vertical cavity surface-emitting laser arrays intended for multimode multicore fiber interconnects. Two circular array designs, differing in their common cathode configuration, are investigated and compared. We demonstrate error-free data transmission (bit-error-rate <;10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-12</sup> ) up to 40 Gb/s and 85 °C for all channels, which can enable an aggregate capacity of 240 Gb/s over a single multicore fiber.

Power-induced polarization switching and bistability characteristics in 1550-nm VCSELs subjected to orthogonal optical injection**Project supported by the National Natural Science Foundation of China (Grant Nos. 61178011, 61275116, and 61475127) and the Natural Science Foundation of Chongqing City, China (Grant No. 2012jjB40011).

The polarization switching (PS) and polarization bistability (PB) characteristics of a 1550-nm vertical-cavity surface-emitting laser (VCSEL) subjected to orthogonal optical injection are systematically investigated. The simulated results show that the PS and polarization-resolved nonlinear dynamical states of the VCSEL are critically dependent on the changing paths of the injected power. The polarization dynamics for different scanning directions of the injected power is presented to explain the polarization evolution during the formation of PS. In the case of forward scanning injected power, with the increase of frequency detuning level between the VCSEL and the injected light, the injected power required for PS gradually increases for negative frequency detuning but exhibits fluctuations for positive frequency detuning. In the case of reversely scanning injected power, the injected power required for PS displays fluctuant changes within the whole frequency detuning range. Specifically, PS may disappear under certain negative frequency detuning and large bias current. Furthermore, the hysteresis width as a function of the frequency detuning is calculated, and the regions for the appearance and disappearance of PB have been determined in the parameter space of the bias current and frequency detuning.

Temperature-Stable 980-nm VCSELs for 35-Gb/s Operation at 85 °C With 139-fJ/bit Dissipated Heat

Energy-efficient oxide-confined 980-nm vertical-cavity surface-emitting lasers (VCSELs) operating at 35 Gb/s at 85 °C with only 139 fJ/bit of dissipated heat are reported. Oxide-aperture-dependent static characteristics and high-speed modulation properties at a high operation temperature of 85 °C are studied in detail. It is demonstrated that oxide-aperture diameters of ~3-4 μm are most suitable for energy-efficient and high bit rate operation at high temperatures. Data transmission experiments for ~3, 3.5, and 4 μm oxide-aperture diameter VCSELs are performed and the results are compared to formulate methods to minimize energy dissipation per bit. To date, our 980-nm VCSELs are the most energy-efficient VCSELs operating at 85 °C at any wavelength.

Single-Mode Vertical Cavity Surface Emitting Laser via Oxide-Aperture-Engineering of Leakage of High-Order Transverse Modes

Properly designed oxide-confined vertical cavity surface emitting laser (VCSEL) allows leakage of the optical modes from the all-semiconductor core region to the selectively oxidized periphery if the orthogonality between the core mode and the modes on the periphery is broken by the oxidation-induced optical field redistribution. The leakage losses are stronger for high-order transverse modes, which have a higher field intensity close to the oxidized region. Single mode lasing in the fundamental mode can thus proceed up to large aperture diameters. The 850-nm GaAlAs thick oxide aperture VCSEL based on this concept is designed, modeled, and fabricated, showing single-mode lasing with the aperture diameters up to 5 μm. Side mode suppression ratio >20 dB is realized at the current density of ~10KA/Cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> in devices with the series resistance of 90 Ω.

Dynamics of 1550-nm VCSELs With Positive Optoelectronic Feedback: Theory and Experiments

Nonlinear dynamics of a 1550-nm vertical-cavity surface-emitting laser with positive optoelectronic feedback are studied both numerically and experimentally. A mapping of dynamical states is presented in the parameter space of feedback delay time and feedback strength, where different states are identified and shown. A bifurcation diagram of the extrema of output peak series versus the feedback delay time is plotted. Various nonlinear dynamical behaviors, including regular pulsing, quasi-periodic pulsing, and chaotic pulsing, have been numerically and experimentally observed. Both numerical simulation and experimental observation indicate that the laser enters a chaotic pulsing state at certain delay times of the feedback loop through a quasi-periodic route.

Effect of the photon lifetime on the characteristics of 850-nm vertical-cavity surface-emitting lasers with fully doped distributed Bragg reflectors and an oxide current aperture

The effect of the photon lifetime in an optical microcavity on the characteristics of 850-nm vertical-cavity surface-emitting lasers (VCSELs) with fully doped distributed Bragg reflectors (DBRs) and an oxide current aperture is studied. The photon lifetime in the microcavity is controlled by varying the upper DBR reflectance. It is found that the speed of VCSELs with a current-aperture diameter of 10 μm is mainly limited by the self-heating effect, despite an increase in the relaxation-oscillation damping coefficient with increasing photon lifetime in the microcavity. At the same time, the higher level of internal optical loss in lasers with a current-aperture diameter of 1.5 μm leads to dominance of the effect of relaxation-oscillation damping independently of the radiation output loss. In the case of devices with a current-aperture diameter of 5.5 μm, both mechanisms limiting the speed operate, which allow an increase in the VCSEL effective modulation frequency from 21 to 24 GHz as the photon lifetime decreases from 3.7 to 0.8 ps.

Impact of the Oxide-Aperture Diameter on the Energy Efficiency, Bandwidth, and Temperature Stability of 980-nm VCSELs

New oxide-confined 980-nm vertical-cavity surface-emitting lasers (VCSELs) with record temperature-stable small-signal bandwidths of 25.6 to 23.0 GHz at 25 to 85 °C are designed, fabricated, and characterized. Technology-based device parameters essential for system-level models of VCSEL-based short-reach and ultrashort-reach optical interconnects are extracted. These parameters include key intrinsic figures-of-merit, including the -3-dB modulation bandwidth, the bandwidth-to-electrical power ratio, and device input impedance, all as functions of temperature, oxide-aperture diameter, and desired range of bias current or current density. Further, the M-factor, relating the intrinsic VCSEL bandwidth to the error-free bit rate for a given external systems configuration and application, is introduced. Our present 980-nm VCSEL technology is capable of 40 Gb/s operation at 85 °C at a simultaneously low current density of 10 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> with an energy of only 100 fJ per bit.

Long-haul dual-channel bidirectional chaos communication based on polarization-resolved chaos synchronization between twin 1550 nM VCSELs subject to variable-polarization optical injection

Based on the polarization-resolved chaos synchronization between twin 1550nm vertical-cavity surface-emitting lasers (VCSELs), a novel long-haul dual-channel bidirectional chaos communication system is proposed. In this system, a time delay signature (TDS)-suppressed chaotic signal, generated by a driving VCSEL (D-VCSEL) under double external cavity feedbacks (DECFs), simultaneously injects into twin VCSELs by variable-polarization optical injection (VPOI) to synchronize them and enhance the chaos output bandwidth of the two VCSELs. The simulated results show that, under proper injection parameters, high-quality polarization-resolved chaos synchronization between the twin VCSELs can be achieved; meanwhile the bandwidths of chaotic signals output from the twin VCSELs have been enhanced in comparison with that of the driven chaotic signal. Based on the high-quality polarization-resolved chaos synchronization, after adopting polarization-division-multiplexing (PDM) and chaos masking (CM) techniques, four 10Gb/s messages hidden respectively in four chaotic carriers can be decrypted effectively after propagating 15km in single-mode fiber (SMF) links. After adopting dispersion-shifted fibers (DSFs) as fiber links, the dual-channel bidirectional chaos communication distance can be extended to 140km.

Impact of the Quantum Well Gain-to-Cavity Etalon Wavelength Offset on the High Temperature Performance of High Bit Rate 980-nm VCSELs

Highly temperature stable, high bit rate oxide-confined vertical-cavity surface-emitting lasers (VCSELs) emitting at 980 nm are presented. Error-free data transmission at 38 Gb/s at 25 °C, 45 °C, 65 °C, and 85 °C is achieved without any change of working point and modulation condition. Static and high-speed properties are analyzed experimentally and theoretically. We numerically investigate the temperature dependence of the differential gain of our quantum well (QW) active region design to explain why a -15-nm QW gain-to-etalon wavelength offset facilitates our 980-nm VCSELs to show simultaneously high bit rate, temperature stability, and energy efficiency. Our VCSELs operate error-free at 42 and 38 Gb/s at 25 °C and 85 °C, respectively, with very low power consumption. Record low 175 fJ of dissipated heat per bit is achieved for 35-Gb/s error-free transmission at room temperature and 177 fJ/bit for 38-Gb/s error-free transmission at 85 °C. Such VCSELs are especially well suited for very-short-reach (<;1 m) optical interconnects in high-performance computers and board-to-board and chip-to-chip integrated photonics.

200-W Operation of an Ion-Implanted Vertical-Cavity Surface-Emitting Laser Array

We have demonstrated a high-power 980-nm vertical-cavity surface-emitting laser diode (VCSEL) array with ion-implanted current apertures. The device was characterized on an exclusively developed water-cooled heat sink for the VCSEL array. A peak output power of over 200 W has been achieved under quasi-continuous-wave operation at a cooling water temperature of 10 °C. The VCSEL array consists of 635 emitters, which were defined by proton implantation and arranged in a closest packed arrangement with 175- \(\mu \) m-spacing in \(5~{\rm mm} \times 5\) mm square. The results provide a chance to the next step for a higher-power VCSEL array.

Integration of 1550nm vertical-cavity surface-emitting laser with gratings on SOI

Abstract We designed a 1550 nm vertical-cavity surface-emitting laser (VCSEL), which comprises a cladding, multiple quantum well (QW) active area, oxide restrict layer, substrate, and high reflectivity distributed Bragg reflectors (DBRs). The VCSEL cavity consists of the cladding, multiple QW active area, and limiting layer. AlGaAsSb/AlAsSb has been used for the DBR mirror system to achieve lattice matching. The QW/barrier comprises GaInAsN/AlGaInAs. By epitaxial growth, the DBR is banded with the active layer at the cavity antinode of the standing wave field, and the cavity length is set to 1.0λ. A double-oxide layer, which is more beneficial than a single-oxide layer, has been adopted in the laser. Discrete Fourier transform is performed on the captured signal to extract the spectral content of the measured signal. By the FDTD method and the PML boundary condition, we achieve a 35% relative light intensity by introducing a TE mode from the cavity to the in-plane grating.