Voltage drop in n- and p-type Bragg reflectors for vertical-cavitysurface-emitting lasers

An analysis of carrier transport in n- and p-type distributed Bragg reflectors (DBR) of vertical-cavity surface-emitting lasers that consist of stacks of quarter-wave GaAs-AlAs layers is presented. The analysis is based on the diffusion-drift approximation with the thermionic boundary conditions at heterojunction interfaces. The spatial distribution of carrier effective masses and mobilities has been taken into account. While the voltage drop in n-type DBR is determined mostly by thermionic emission at the interfaces, the drift-diffusion component of the voltage drop is comparable with the thermionic emission in p-type DBR. We present the calculated resistance as a function of graded-region thicknesses and doping levels, which can be useful for low-resistive DBR design     

Improved semiconductor-laser dynamics from induced population pulsation

This paper investigates theoretically the modification of dynamical properties in a semiconductor laser by a strong injected signal. It is found that enhanced relaxation oscillations are governed by the pulsations of the intracavity field and population at frequencies determined by the injected field and cavity resonances. Furthermore, the bandwidth enhancement is associated with the undamping of the injection-induced relaxation oscillation and strong population pulsation effects. There are two limitations to the modulation-bandwidth enhancement: Overdamping of relaxation oscillation and degradation of flat response at low frequencies. The injected-laser rate-equations used in the investigation reproduce the relevant aspects of modulation-bandwidth enhancement found in the experiment on injection-locked vertical-cavity surface-emitting lasers.     

Tunable VCSEL

Vertical-cavity surface-emitting lasers (VCSELs) are now key optical sources in optical communications. Their main application is currently in local area networks using multimode optical fibers. VCSELs are also being rapidly commercialized for single-mode fiber metropolitan area and wide area network applications. The advantages of VCSEL include simpler fiber coupling, easier packaging and testing, and the ability to be fabricated in arrays. In addition, VCSELs have an inherent single-wavelength structure that is well suited for wavelength engineering. All these advantages promise to lead to cost-effective wavelength-tunable lasers, which are essential for the future intelligent, all-optical networks. The author reviews the advances on wavelength-tunable VCSELs. She summarizes some of the early breakthroughs in wavelength engineering of VCSELs and then concentrates on the designs and properties of micromechanical tunable VCSEL     

Monolithic multiple wavelength surface emitting laser arrays

A two-dimensional (2-D) surface emitting laser array emitting 140 unique, nonredundant, uniformly separated, single-mode wavelengths in the 980-nm regime is described. The wavelength separation between neighboring lasers is as small as 0.3 nm. A large total wavelength span of 43 nm was obtained without compromising the performance of the lasers. All 140 lasers have nearly the same threshold currents, voltages, and resistances. The techniques used are generic and can be readily extended to both longer and shorter wavelength lasers. The authors also report the first wavelength division multiplexing system experiment using part of this laser array. A BER (bit-error ratio) of 10 ^-9 at 155 Mb/s was obtained with simultaneous operation of four lasers at a wavelength separation of 1.5 nm. Negligible optical and electrical crosstalk was observed between the lasers     

Wavelength shift in vertical cavity laser arrays on a patternedsubstrate

The authors demonstrate a spatially chirped emission wavelength in vertical cavity surface emitting laser (VCSEL) arrays grown by molecular beam epitaxy. The wavelength shift is due to a lateral thickness variation in the Al_0.2Ga_0.8As cavity, which is induced by a substrate temperature profile during growth. A 20 nm shift in lasing wavelength is obtained in a VCSEL array     

Buried selectively oxidized AlGaAs structures grown on nonplanar substrates

We demonstrate a novel buried oxide-grating structure formed by selectively oxidized Al/sub x/Ga/sub 1-x/As grown on nonplanar substrates using low-pressure metal-organic chemical vapor deposition (MOCVD) for the first time. Localized aluminum content variation in AlGaAs is obtained with MOCVD growth on nonplanar substrate. Buried aluminum oxide/semiconductor-distributed-feedback structure is achieved with selective oxidation of these AlGaAs layers. We fabricated a resonant-cavity-enhanced photodetector with the imbedded buried-oxide structure and measured the photodetector responsivity spectrum.     

Dynamic, polarization, and transverse mode characteristics ofvertical cavity surface emitting lasers

The dynamic, polarization, and transverse mode characteristics of strained InGaAs-GaAs quantum well vertical cavity surface emitting lasers (VCSELs) emitting at 0.98 μm are investigated. The dynamic behavior of VCSELs with high and low operating voltages and series resistances is compared. A large wavelength chirp in the lasing spectrum was observed for the lasers with high voltage/resistance, even under low-duty-cycle pulse operation. This is thought to be due to resistive heating close to the laser junction. It is observed that the transverse mode structure of VCSELs and their dependence on laser dimensions and drive current are highly analogous to those of edge emitting lasers, whereas the polarization characteristics of the two types of lasers are significantly different     

Temperature dependence of light-current characteristics of0.98-μm Al-free strained-quantum-well lasers

The decrease of the differential efficiency of 0.98-μm semiconductor lasers with temperature can make high power, high temperature applications difficult. We present an experimental and theoretical study of the temperature dependence of the internal quantum efficiency, internal loss and differential gain of 0.98-μm InGaAs/InGaAsP/InGaP strained quantum well lasers. In contrast to some earlier results, our measurements show the dominance of internal loss, attributed to free carrier absorption, in determining the temperature dependence of the differential efficiency, and show that leakage current is negligible below 120°C     

Parasitics and design considerations on oxide-implant VCSELs

Vertical-cavity surface-emitting lasers (VCSELs) are the enabling technology for low-cost high bit rate communication systems. However, the device parasitics typically limit the high-speed performance. In this letter, the modulation responses of oxide-confined and oxide-implant VCSELs are compared. Both of the configurations were fabricated side by side on the same sample and were thoroughly examined. In oxide-implant VCSELs, the low frequency parasitic rolloff is removed by additional implantation. A VCSEL radiatively coupled model is shown to match the experimental data very well without any fitting parameters. This model can be used to design the device geometry and eliminate the parasitic limitation on high-speed performance     

28 GHz optical injection-locked 1.55 /spl mu/m VCSELs

A record resonance frequency of 28 GHz and an intrinsic laser 3 dB bandwidth of 34 GHz is reported for a directly modulated injection-locked 1.55 /spl mu/m VCSEL. The small-signal modulation response is experimentally investigated using polarisation-maintaining components.