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

1.3 to 1.55 $\mu\hbox{m}$ All-Optical Wavelength Conversion for VCSEL-to-VCSEL Inversion and Logic

We demonstrate all-optical wavelength conversion from the O to the C band using a 1550-nm commercial vertical-cavity surface-emitting laser (VCSEL) subject to external optical injection at 1300 nm. For the first time to our knowledge, we show that this configuration can also be used to develop all-optical inverters and cross-band NAND and NOR gates based on VCSEL devices, and investigate their potential speed of operation.

Temperature Dependent Dynamics in a 1550-nm VCSEL Subject to Polarized Optical Injection

We report the first comprehensive experimental analysis on the dynamics of an optically injected 1550-nm vertical-cavity surface-emitting laser (VCSEL) over a wide temperature range going from -20°C to 60°C . We show the variations with temperature on the measured stability maps of the 1550-nm VCSEL when subject to parallel and orthogonal polarized optical injection and for different applied bias currents above threshold. The change in the shape of the stable locking region with temperature with the device subject to parallel polarized optical injection is attributed to the change of linewidth enhancement factor (α). In addition, we also show the changes of the polarization switching and locking regions with temperature for the case of orthogonal polarized optical injection.

Polarization Bistability Induced by Orthogonal Optical Injection in 1550-nm Multimode VCSELs

We have analyzed experimentally the optical power polarization bistability induced in a long-wavelength multitransverse mode vertical-cavity surface-emitting laser (VCSEL) under orthogonal optical injection. Bistability with very wide hysteresis cycles is measured for both the fundamental and the high-order transverse mode of the device. The shape of the associated hysteresis cycle is different for each of the two transverse modes. The power of the parallel polarized high-order (fundamental) transverse mode remains constant (gradually decreases) as the injected power is increased. Both powers suddenly drop to low levels at large values of the injected power. The squared shape measured for the I/O characteristic of the high-order transverse mode is of particular interest for obtaining good quality all-optical inversion and all-optical regeneration for use in optical telecommunication networks.

85 °C error-free operation at 38 Gb/s of oxide-confined 980-nm vertical-cavity surface-emitting lasers

Extremely temperature stable oxide-confined high-speed 980-nm vertical-cavity surface-emitting lasers (VCSELs) for optical interconnects are presented. Error-free performance at 38 Gb/s and 40 Gb/s is demonstrated at temperatures as high as 85 °C and 75 °C, respectively. No adjustment of driving conditions was found to be necessary from room temperature up to 85 °C. In addition, energy-efficient 35 Gb/s operation at a very low pump current of only 4 mA is demonstrated with a low dissipated heat-to-bit rate ratio of 233 mW/Tbps. These are by far the highest bit rates reported for VCSELs at such temperatures.

Power and spectra polarization of large-aperture rectangular-shaped vertical-cavity top-emitting lasers

980-nm large-aperture top-emitting vertical-cavity surface-emitting lasers (VCSELs) were investigated through introducing the rectangular post. Both H-polarization (horizontal) and V-polarization (vertical) coexisted in the rectangular-shaped large-aperture VCSELs. Under the entire range of operation current, H-polarization dominated over V-polarization which was parallel to the shorter side of the rectangular output aperture. It was found that spectrum blue-shift of H-polarization light occurred with respect to V-polarization light at three different aspect ratios, and this can be explained by the dependence of longitudinal propagation constant on the aspect length. The rectangular post structure was found to be effective for polarization stabilization in broad-area VCSELs without serious degradation of light output characteristics.

Rare disruptive events in polarisation-resolved dynamics of optically injected 1550 nm VCSELs

Presented are experimental observations on the occurrence of rare disruptive events in the polarisation-resolved dynamics of 1550nm vertical cavity surface-emitting lasers (VCSELs) when subject to orthogonally-polarised optical injection. These events, characterised by high amplitude pulses, appeared individually and simultaneously in the two orthogonal polarisations of the fundamental transverse mode of the device. They could be observed in different regions of dynamic behaviour, including stable injection locking and chaos.

99 fJ/(bit$\cdot$km) Energy to Data-Distance Ratio at 17 Gb/s Across 1 km of Multimode Optical Fiber With 850-nm Single-Mode VCSELs

We present extremely energy-efficient oxide-confined 850-nm single-mode vertical-cavity surface-emitting lasers (VCSELs) for optical interconnects. Error-free transmission at 17 Gb/s across 1 km of multimode optical fiber is achieved with an ultra-low energy-to-data ratio of 99 fJ/bit, corresponding to a record-low energy-to-data-distance ratio of 99 fJ/(bit ·km). This performance is achieved without changing any of the driving parameters up to 55 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C. To date our VCSELs are the most energy-efficient directly modulated light-sources for data transmission across all distances up to 1 km of multimode optical fiber.

Polarization stable vertical-cavity surface-emitting laser with surface sub-wavelength gratings

By introducing sub-wavelength grating into an oxide-confined 980-nm vertical-cavity surface-emitting laser (VCSEL), the VCSEL with a stable polarization is realized. A sub-wavelength zero-order grating period is smaller than the wavelength in the semiconductor medium. This choice avoids a reduction in efficiency due to higher diffracting light. After the grating is etched, the threshold gains of E-fields parallel and perpendicular to the grating groves direction are different. The polarization therefore would be controlled. Good laser characteristics are obtained: the orthogonal polarization suppression ratio is over 12 dB and the threshold current increases just 7.14%.

980-nm High-Power Low-Divergence VCSELs Achieved by Optimization of Current Density Distribution

In this paper, we report the design and fabrication of 980-nm vertical-cavity surface-emitting lasers and arrays that provide high output power and low divergence angle without any additional collimating optics. The influence of the structure of the contact on the current density distribution inside the active region was analyzed using the 3-D finite-element method. Uniform current distribution is achieved by optimizing the diameter of the p-contact, and the consequent improvement in beam divergence and thermal behavior is shown. A low divergence angle of 5.4° is obtained for a single device with continuous-wave (CW) power of 1.46 W at room temperature. The 8 × 8 array shows a divergence angle of 10.2° at 4 A with a CW power of 1.95 W. In addition, the measured results were compared with the results from devices without optimization of current density distribution.

Impact of Photon Lifetime on High-Speed VCSEL Performance

We investigate the impact of reduced photon lifetime on the static and dynamic performance of high-speed, oxide-confined 850-nm vertical-cavity surface-emitting lasers (VCSELs). The photon lifetime is reduced by a shallow-surface etch that lowers the reflectivity of the top mirror. From an analysis of the dependence of slope efficiency on mirror loss (etch depth) and temperature, we deduce values for the internal quantum efficiency and the internal optical loss and their dependencies on temperature. From an analysis of the dependence of the small-signal-modulation response on photon lifetime (etch depth) and temperature, we deduce values for differential gain and gain compression, and their dependencies on photon lifetime and temperature. We find a tradeoff between high resonance frequency and low damping for speed enhancement, leading to an optimum photon lifetime close to 3 ps for this particular VCSEL design that enables a modulation bandwidth of 23 GHz and error-free transmission at 40 Gb/s.

Dynamics of Polarized Optical Injection in 1550-nm VCSELs: Theory and Experiments

We report novel theoretical results obtained from combining the method of largest Lyapunov exponent (LLE) with the spin-flip model (SFM), including noise, to model a vertical-cavity surface emitting laser (VCSEL) subject to polarized optical injection. The LLE is applied to the numerical solutions in order to automatically calculate stability maps that characterize the dynamics. The SFM has been extended and generalized to allow for optical injection of arbitrary polarization. Measurements on a 1550-nm VCSEL have been used to estimate the values of key parameters for use in the model and with these we demonstrate excellent agreement between theory and experiment.

1550-nm High-Speed Short-Cavity VCSELs

The latest advances in 1550-nm high-speed short-cavity vertical-cavity surface-emitting lasers (VCSELs) are presented. The electrical, optical, and thermal design of these devices is discussed with respect to their high-speed modulation behavior. The implemented design improvements allow high output powers for ambient temperatures up to 90°C, high differential quantum efficiencies up to 40%, temperature-stable single-mode emission, low thermal resistances down to 1.6 K/mW, small-signal modulation bandwidths in excess of 17 GHz, and error-free data transmission at room temperature at data rates of 35 Gb/s in back-to-back configuration and up to 25 Gb/s over 4.2 km of single-mode fiber.

Assessment of VCSEL thermal rollover mechanisms from measurements and empirical modeling

We use an empirical model together with experimental measurements for studying mechanisms contributing to thermal rollover in vertical-cavity surface-emitting lasers (VCSELs). The model is based on extraction of the temperature dependence of threshold current, internal quantum efficiency, internal optical loss, series resistance and thermal impedance from measurements of output power, voltage and lasing wavelength as a function of bias current over an ambient temperature range of 15-100 °C. We apply the model to an oxide-confined, 850-nm VCSEL, fabricated with a 9-μm inner-aperture diameter and optimized for high-speed operation, and show for this specific device that power dissipation due to linear power dissipation (sum total of optical absorption, carrier thermalization, carrier leakage and spontaneous carrier recombination) exceeds power dissipation across the series resistance (quadratic power dissipation) at any ambient temperature and bias current. We further show that the dominant contributors to self-heating for this particular VCSEL are quadratic power dissipation, internal optical loss, and carrier leakage. A rapid reduction of the internal quantum efficiency at high bias currents (resulting in high temperatures) is identified as being the major cause of thermal rollover. Our method is applicable to any VCSEL and is useful for identifying the mechanisms limiting the thermal performance of the device and to formulate design strategies to ameliorate them.

Polarization and Time-Resolved Dynamics of a 1550-nm VCSEL Subject to Orthogonally Polarized Optical Injection

We report a first experimental study of the simultaneous temporal and polarization-resolved dynamics of a 1550-nm vertical-cavity surface-emitting laser (VCSEL) subject to orthogonally polarized optical injection. A novel technique is used to reveal the behavior of both polarizations at the VCSEL's output. In general, the same type of dynamics can be seen in both polarizations of the fundamental transverse mode but with unequal intensity/modulation depth. We show that both polarizations exhibit period one and chaotic dynamics and reveal antiphase dynamics. These results offer promise for the development of dual-channel, anticorrelated periodic and chaotic-signal generators with a single VCSEL for use as oscillators or chaotic sources in present and future optical networks.

Intensity Noise Characteristics of Multimode VCSELs

The intensity noise spectra of several multi-transverse mode 1550-nm vertical-cavity surface-emitting lasers (VCSELs) have been experimentally investigated. For a VCSEL emitting in two parallel polarized transverse modes the noise spectra of the individual modes and total power show two resonance peaks. The frequencies at which both peaks appear have been measured as a function of the bias current. Our experimental results confirm the theoretical predictions of Valle Differences in the noise spectra are experimentally observed when considering a VCSEL emitting in three transverse modes with polarization instabilities. We show that additional peaks appear in the noise spectra of the total power as the bias current is increased.

Effect of SiO2 Reflection Reducing Coating on the Vertical Cavity Surface Emitting Laser

The effects of epitaxis SiO2 film on the characteristics of DBR reflectivity were simulated, and the reflectivity of graded interface DBR is lower than the abrupt one was studied. 850nm vertical-cavity surface-emitting lasers were fabricated, the DBR is formed by graded heterojunction of AlxGa1–xAs, the top and bottom DBR have 22 and 34 pairs of mirror, and current was confined by oxide aperture, and then different thickness of SiO2 film were grew on the top DBR. and found that a certain thickness (λ/4) of the SiO2 reflection reducing coating could make the vertical-cavity surface-emitting lasers output power increased about 3–5 mW, the threshold current does not increase obviously, the reason is external quantum efficiency increases more than the threshold current. So the SiO2 reflection reducing coating does not affect the lasing of lasers and threshold current obviously, but It can significantly improve the output power. The experimental results agree well with the theoretical expectation.

Graded index profiles and loss-induced single-mode characteristics in vertical-cavity surface-emitting lasers with petal-shape holey structure

The 850-nm oxide-confined vertical-cavity surface-emitting lasers with petal-shape holey structures are presented. An area-weighted average refractive index model is given to analyse their effective index profiles, and the graded index distribution in the holey region is demonstrated. The index step between the optical aperture and the holey region is obtained which is related merely to the etching depth. Four types of holey vertical-cavity surface-emitting lasers with different parameters are fabricated as well as the conventional oxide-confined vertical-cavity surface-emitting laser. Compared with the conventional oxide-confined vertical-cavity surface-emitting laser without etched holes, the holey vertical-cavity surface-emitting laser possesses an improved beam quality due to its graded index distribution, but has a lower output power, higher threshold current and lower slope efficiency. With the hole number increased, the holey vertical-cavity surface-emitting laser can realize the single-mode operation throughout the entire current range, and reduces the beam divergence further. The loss mechanism is used to explain the single-mode characteristic, and the reduced beam divergence is attributed to the shallow etching. High coupling efficiency of 86% to a multi-mode fibre is achieved for the single-mode device in the experiment.

Low power penalty tunable slow light using vertical-cavity surface-emitting laser amplifier

A tunable slow light of 2.5-Gb/s pseudo-random binary sequence signal using a 1550-nm vertical-cavity surface-emitting laser (VCSEL) is experimentally demonstrated. The influences of the bias current and the gain saturation on the slow light are investigated. With bias current increasing, tunable optical group delay up to 98 ps is obtained at room temperature. Demonstration of the time delay between 16 and 24 ps by signal intensity change is reported. Under an appropriate bias current, by tuning the input signal to track the peak gain wavelength of the VCSEL, slow light of a power penalty as low as 1 dB is achieved. With such a low power penalty, the VCSEL has a great potential application as a compact optical buffer.

MBE growth of VCSELs for high volume applications

Mass market applications like laser computer mouse or optical data transmission based on vertical-cavity surface-emitting laser (VCSEL) chips need a high over all yield including epitaxy, processing, dicing, mounting and testing. One yield limitation for VCSEL structures is the emission wavelength variation of the substrate surface area leading to the fraction on laser chips which are below or above the specification limits. For most 850nm VCSEL products a resonator wavelength variation of ±2nm is common. This represents an average resonator thickness variation of much less than 1% which is quite challenging to be fulfilled on the entire processed wafer surface area. A high over all yield is demonstrated on MBE grown VCSEL structures.

High-Performance Zn-Diffusion 850-nm Vertical-Cavity Surface-Emitting Lasers With Strained InAlGaAs Multiple Quantum Wells

We demonstrate a high-performance Zn-diffusion 850-nm vertical-cavity surface-emitting laser (VCSEL). By the use of strained InAlGaAs/AlGaAs multiple quantum wells for the active region, our structure can have a much higher maximum output power, higher differential quantum efficiency (DQE), and larger modulation current efficiency (D-factor) than those of non-strained control GaAs/AlGaAs VCSELs. Two different Zn-diffusion depths were adopted in our devices with the same single-oxide current-confined aperture (~6 μm) to further optimize the static and dynamic performance, respectively. The device with a deep Zn-diffusion depth (~1.2 μm) shows an optimized static performance, which includes a low threshold current (0.8 mA), high DQE (90% at ~1.2 mA), and a maximum output power as high as 9.7 mW. On the other hand, the device with a shallow Zn-diffusion depth (<; 0.6 μm) demonstrates good dynamic performance and exhibits a large D-factor (9.5 GHz/mA1/2), high maximum data rate (32 Gbit/s error-free) performance, and very-high data-rate/power-dissipation ratio (5.25 Gbit/s/mW) under an extremely small driving voltage (Vpp: 0.25 V).