Each year, more VCSEL technologies make the transition from research curiosities to commercially available products. In this paper we describe several such technologies at Honeywell, each at a different stage of that transition. Oxide-confined devices are already past the transition stage. We describe the generally excellent reliability of oxide-confined devices already in high-volume production, and compare it to results of the most recent-and possibly last-long-term reliability study of proton-implanted VCSELs. We report on detailed package-VCSEL interaction modeling, which is being used to improve performance and extend the life of common form-factor packages. We also note Honeywell's progress toward commercialization of VCSELs and allied products at wavelengths other than 850 nm.
TrueLight Corporation was found in 1997 and it is the pioneer of VCSEL components supplier in Taiwan. We specialize in the production and distribution of VCSEL (Vertical Cavity Surface Emitting Laser) and other high-speed PIN-detector devices and components. Our core technology is developed to meet blooming demand of fiber optic transmission. Our intention is to diverse the device application into data communication, telecommunication and industrial markets. One mission is to provide the high performance, highly reliable and low-cost VCSEL components for data communication and sensing applications. For the past three years, TrueLight Corporation has entered successfully into the Gigabit Ethernet and the Fiber Channel data communication area. In this paper, we will focus on the fabrication of VCSEL components. We will present you the evolution of implanted and oxide-confined VCSEL process, device characterization, also performance in Gigabit data communication and the most important reliability issue
Group III-Nitride-Arsenides are promising materials for 1.3 micron opto-electronic devices grown on GaAs substrates, allowing AlAs/GaAs distributed Bragg reflector (DBR) mirrors and integration with GaAs electronics. Nitrogen decreases the GaAs bandgap dramatically, and the smaller GaN lattice constant results in less strain in GaInNAs compared to InGaAs.
However, the anneal necessary to achieve device quality material shifts the emission peak to shorter wavelengths. Secondary ion mass spectroscopy (SIMS) depth profiling on GaInNAs quantum wells shows that nitrogen diffusion exceeds indium diffusion during anneal. We have demonstrated broad-area lasers, pulsed lasers, and CW VCSELs. However, due to nitrogen out-diffusion from the QWs, the operating wavelength of these initial devices was shorter than 1.23µm.
Subsequent use of GaNAs barriers surrounding the QWs reduced the shift of the emission peak during anneal, as the GaAsN diffused nitrogen into the QW. This also resulted in longer wavelength emission due to decreased electron confinement energy and compensted overall strain. This new active region resulted in devices emitting at 1.3 micron. The new design also improved laser characteristic temperature T0 from 105K to 146K for similar devices.
We present an accurate experimental characterization of the dynamical properties of polarization switching (PS) in single transverse and longitudinal mode vertical-cavity surface-emitting lasers (VCSELs). When a VCSEL is driven with a constant current at its polarization switching point, it makes random jumps between its two linear polarization states. This phenomenon is called mode-hopping. The permanence times in the two polarization states show an exponentially decreasing distribution, according to Arrhenius? law. The average permanence time varies over several orders of magnitude depending on the relative difference between threshold and switching current. We have performed a statistical experimental characterization of the residence times of mode hopping VCSELs for both proton implanted and oxide confined samples, and find our results to be in excellent agreement with the theoretical predictions from a novel intensity rate equation model.
Polarization selection in small-area vertical-cavity
surface-emitting lasers (VCSELs) is studied experimentally in
dependence on injection current and substrate temperature in the
vicinity of the minimum threshold condition. Different types of
polarization switching (PS) are observed and analyzed: PS from the
high to the low frequency mode, PS from low to high frequency and
double switching (from high to low and back to high frequency).
Whereas PS from the high to the low frequency mode is due to a
change of sign of linear dichroism, the optical spectra show
dynamical transition states for the other case which hint to the
relevance of nonlinear effects. A comparison to the predictions of
the SFM-model based on phase-amplitude coupling is given. The
interplay of spatial and polarization effects can depend strongly
on the ambient temperature.
We demonstrate experimentally how an external mechanical stress applied to a VCSEL wafer results in an in-plane anisotropic strain, which dramatically alters the polarization behavior of our VCSELs. In the presence of in-plane strain, the VCSEL still emits linearly polarized light but its direction strongly depends on the magnitude and the orientation of the strain (with respect to the crystal co-ordinate system). The latter behavior can be understood by taking into account the elasto-optic effect and the proper strain distribution. Furthermore, for a specific range in magnitude and orientation of the externally induced strain, current induced polarization switching between the two fundamental modes (with orthogonal linear polarization) is present in a reproducible way. The current at which switching occurs strongly depends on the magnitude of the external stress and can be tuned in the whole region of single-mode operation.
These effects can be explained by accounting for the anisotropy of the valence band. The latter is induced by the in-plane uniaxial strain, leading to a modification of density of states and effective masses for different directions in the plane of the QW, and henceforth to a gain anisotropy and a different gain curve for each of the two polarization modes. Furthermore, the frequency splitting between the two cavity eigenmodes (also a result of the stress via the elasto-optic effect) has to be taken into account. We will discuss how the gain anisotropy changes with current, lattice temperature and carrier density, and how all these determine the polarization behaviour of VCSELs.
We have successfully developed a selective surface coating technique to control the modal behavior of the ionimplanted vertical cavity surface emitting laser. Using selective deposition of germanium coating by lift-off process, we could spatially control the threshold gain condition of the VCSEL to support the single transverse mode. The threshold current is 7 mA and single transverse mode operation is maintained up to 1 mW. The method is simple and nondestructive compared to other techniques.
We present an approach for systematic high-speed characterization of VCSELs and discuss both its potential benefits and problems. We show how the VCSEL dynamics, under certain conditions, can be well described by a small number of key parameters that can be extracted from measurements and used for further optimization. The calibrated small signal modulation responses of the laser are measured and fitted to an analytical transfer function allowing the estimation of the resonance frequency, damping factor and parasitic cut-off at different bias points. From this data the relative importance of different bandwidth limiting effects due to damping, thermal heating and parasitics can be deducted. We illustrate the approach on 850nm datacom VCSELs using either ion implantation, selective oxidation or semi-insulating regrowth for current confinement. The bandwidth ofthe implanted device appears to be limited by parasitics effects to 3.3GHz. Due to a much smaller injection diameter, the oxidized VCSEL reaches 10GHz, being mainly limited by the high damping. Finally the regrown VCSEL operates up to 5GHz, limited by the parasitics
High bit rate communication links are placing increasing demands on the performance and cost of semiconductor laser diodes. VCSELs are uniquely suited to meeting the requirements of 10 Gb/s and higher applications. The laser requirements include high temperature operation, high bandwidth, high reliability, short rise and fall times, low RIN, low jitter, low RMS linewidth, and drive circuit compatibility. We will discuss the major challenges to achieving these goals as well as approaches that have been successful to date.
MITEL Semiconductor is developing the next generation low cost, high performance transceivers for data communication. The increasing quantity of data being transferred over the Internet demands very high capacity interconnects. A low cost, high-performance alternative is the use of parallel fiber interconnects where the light is, for example, coupled into a 12channel fiber-ribbon. Parallel interconnects require good uniformity in order to reduce escalating costs and complexity. In this paper we report on the static and the modulation properties of 850nm multimode oxide VCSELs for use in such Gb/s transceiver system. Static power-current-voltage characteristics with good uniformity were obtained for different structures, with threshold currents down to sub-mA. A maximum small signal 3-dB bandwidth of 10 GHz and a modulation current efficiency up to 8.4 GHztsJ[rnA] were measured. Single channel results are presented for VCSELs operated at data rates from 2.5-10Gb/s.
We present top surface emitting two- and three-stage bipolar
cascade vertical cavity surface emitting lasers (VCSELs) for
continuous wave (cw) operation at room temperature. The results
are compared with a conventional VCSEL grown under identical
growth conditions for reference. The improvement of both threshold
current density and differential quantum efficiency can be clearly
shown in structures with unchanged top mirror reflectivities (19
pairs). To further increase the differential quantum efficiency, a
device with only 14 p-type top mirror pairs is fabricated,
yielding an efficiency of 130% for cw room temperature
operation. From noise measurements, we extract a modulation
efficiency of 8.2 GHz/sqrt(mA}, indicating favorable dynamic
properties of these VCSEL.
The study of the lineshape of semiconductor lasers is very interesting, being related to phase and frequency noise sources which are usually hidden in other kinds of laser. The importance of this topic for Vertical Cavity Surface Emitting Lasers (VCSELs) is further increased by their large impact in communication applications, since frequency and phase noise limits the performances of different optical communication techniques.
We have performed such a study on an air-post AlAs/AlGaAs VCSEL. We have recorded the lineshape at different injection current levels by heterodyne with a narrow linewidth extended-cavity laser, while the frequency noise spectrum is investigated using a Fabry-Perot cavity as frequency discriminator.
In single-mode emission conditions, the lineshape is Lorentzian at low pump current, while a Gaussian contribution is evident for higher pump level. The Lorentzian linewidth is inversely proportional to the laser power and can be compared with the results of the Schawlow-Townes-Henry theory properly modified to consider the particular structure of VCSELs.
The study of the frequency noise shows that a quasi-Gaussian lineshape
is due to an excess low frequency noise. This contribution has a
1/f^n dependence, with n around 1, for frequencies higher than
20 kHz and is flat at lower frequencies. This peculiar power spectrum has been observed in the electric noise of AlGaAs Bragg reflectors. The current noise generated in the Bragg mirrors is the source of the frequency noise through the fluctuations of the cavity optical length.
The results are extended to other kinds of VCSELs.
VCSEL arrays are attractive low-cost high-speed sources for free space and fiber coupled links. An overview of existing device types and technologies as well as trends in device technology, optical interconnect and parallel optical datacom link applications is given. We discuss applications and performance limits of 1D and 2D VCSEL arrays. At Avalon Photonics Ltd. a broad variety of devices (up to 16x16) is developed and fabricated. Top and bottom emitting arrays offer new perspectives in optical and electrical packaging and are therefore of high interest for short haul parallel optical links, board-to-board and on-board ultra high-speed optical interconnects. Bandwidth requirements are increasing rapidly. The limits of VCSEL arrays in terms of modulation speed are analyzed and RF device optimization is discussed. Device and package limitations have to be considered. Important system level implications are also pointed out. Results of selectively oxidized 850nm multi transverse mode VCSEL arrays demonstrating low threshold current, high modulation efficiency and excellent multi-gigabit performance are presented. The paper ends with a discussion of future prospects in the field.
Coupled arrays of vertical-cavity surface-emitting lasers were realized by patterning the reflectivity of the top-distributed Bragg reflector using a phase-matching layer and a metal grid. For improved current injection and better heat dissipation the devices were selectively oxidized. Continuous-wave room-temperature operation of these anays has been achieved at 960 urn. Polarization resolved measurements revealed a stable behavior without any polarization flips in the fundamental lasingmode regime. Spectrally resolved measurements of the near- and far-field showed an abrupt transition from the fundamental super-mode with a TEM00-shaped near-field in every individual pixel with a beam divergence of 5.5°in both arraydirections to a TEM01-like super-mode with 5.5° beam divergence in one direction and 1 1° in the other direction. Measurements with high angular resolution showed full-width at half-maximum of the far-field intensity lobes as small as 0.89° for a lOxlO array.
In the implementation of optical data links, issues of power consumption, bandwidth and sensitivity have to be addressed in the design of optoelectronic components. This is especially important in high density parallel applications where large amounts of heat can cause thermal problems and performance degradation. We present two-dimensional vertical-cavity surface-emitting laser (VCSEL) arrays for high density optical interconnects.
Devices emit at 850 or 980\,nm wavelength and exhibit excellent operation data. The VCSEL chips are used - within the framework of the European research project OIIC - in multiple CMOS-to-CMOS link demonstrators.
In this paper we discuss a 0.6 µm-CMOS VCSEL transmitter with 32 data
channels whose power consumption is only 15.7 mW/ch at 1 Gb/s/ch data transmission.
Now we have studied the development of the optical devices used in optical wireless communication systems. For optical wireless systems, the emitted light should have an intensity distribution in the shape of a pill-box. Use of VCSEL array was believed to allow the emitted light to have pill-box distribution and we performed the study concerning the optimum VCSEL array for optical wireless systems. This article describes the development of the VCSEL array for optical wireless systems.
Red VCSELs for emission wavelengths near 650nm find applications in emerging technologies such as plastic-fiber-based data communication. However, these devices are challenging due to low band offsets and high electrical and thermal resistivity of especially the p-DBR. The paper presents the optimization of p-DBR and QW design for the reduction of the series resistance and the threshold current density.
VCSEL structures were grown using MOVPE and processed to air-post mesas. The resistance of the p:DBR mirrors was optimized using different dopants and interfaces. By changing from Zn doping and abrupt interfaces to the dopant C and introducing graded interfaces the differential resistance decreased. Due to a relative shift across the wafer between the DBR stop-band and gain peak wavelength defined by the MQW active region, devices are available with lasing wavelengths between 638nm and 662nm in pulsed-mode operation. Threshold current densities of 3.6kA/cm2 at 650nm are measured.
For improving device parameters a current aperture was processed by selective wet oxidation of AlxGa1-xAs with varying x. Cw laser operation is achieved for wavelengths between 644nm and 657nm at 10°C ambient temperature. With threshold currents under 4mA maximum cw output powers of 160µW are obtained at wavelengths of 657nm and 650 nm.
Visible vertical-cavity surface-emitting lasers (VCSELs) are potential light sources for polymer optical fibre (POF) data transmission systems. Minimum attenuation of light in standard PMMA-POFs occurs at about 650 nm. For POFs of a few tens of meters in length VCSELs at slightly longer wavelengths (670 - 690 nm) are also acceptable. So far, the visible VCSELs have been grown by metal organic chemical vapour deposition (MOCVD). They may also be grown by a novel variant of molecular beam epitaxy (MBE), a so-called all-solid-source MBE or SSMBE. In this paper, we describe growth of the first visible-light VCSELs by SSMBE and present the main results obtained. In particular, we have achieved lasing action at a sub-milliamp cw drive current for a VCSEL having the emission window of 8um in diameter, while a 10um device exhibited an external quantum efficiency of 6.65% in CW operation at room temperature. The lasing action up to temperature of 45°C has been demonstrated.
A selectively oxidized Vertical Cavity Surface Emitting Laser (VCSEL) has been designed and fabricated for operation at a wavelength of l.546im. The lattice matched device structure was grown on an InP substrate using 111-V quaternary semiconductor alloys for Bragg mirrors and GaInAsP-based unstrained Multi-Quantum Wells (MQW) for the active layer. The mirror reflectivities are 97% for the top Distributed Bragg Reflector (DBR) consisting of 16 pairs of AlGaInAs/InP layers, and 99.9% for the bottom DBR consisting of22 pairs. A threshold current as low as 2.2mA has been achieved. The threshold voltage was typically lower than 2.0 V and the power output exceeded 1mW. The laser spectrum from a 7jtm confmed diode shows a single mode ofoperation at 1.54 m. The single fundamental mode was present at all current levels. The influence ofthe intentional and growth-related compositional grading at the heterointerfaces as well as random and fixed thickness variations of layer thickness on the mirror reflectivity and laser characteristics has been investigated, and key sensitivities to laser performance have been determined through computational simulations. It is shown that the degree of surface roughness and random thickness variation have the strongest impact on the device performance.
Operation of IV-VI vertical-cavity surface-emitting lasers emitting in the mid-infrared is reported. The microcavity laser structures were grown by molecular-beam-epitaxy on BaF2(111) substrates. The stimulated emission is generated in PbTe quantum wells embedded in high finesse Pb0.95Eu0.05Te/EuTe microcavity structures by optically pumping with fs and ns laser pulses. Laser samples were designed for operation at 2 K, at 70 K and at room temperature with emission at 6 mm, 4.8 mm and around 3 mm, respectively. At a wavelength of 3.1 mm, laser operation is obtained up to a temperature of 65°C, limited by nonradiative recombination processes. The temperature dependence of the emission is explained in terms of the strong temperature dependence of the energy band gap of the lead salt compounds as compared to the microcavity resonance energy in agreement with envelope function calculations of the quantum well energy levels.
This talk will discuss the state-of-the-art VCSEL-based solutions that are currently being investigated for next-generation central-office and local-area network applications. Serial 10 Gb/s VCSELs, VCSEL-based four-wavelength coarse-wavelength division multiplexing (CWDM) and multichannel (parallel-optic) solutions are described in more detail. Specific characterization data will be provided on Alvesta four-channel transceiver solution.
Recently VCSEL technology has been emerging outside data communications. VCSEL's unique characteristics of low power consumption, high efficiency, circular beam spot, and low manufacturing cost make it a good candidate to replace LEDs or edge-emitting diode lasers in optical sensors. Optical sensors based on VCSEL technology in industrial applications have been studied in Honeywell. Traditional interruption type and Moire fringes type VCSEL sensors have demonstrated high resolution, simple design and lower cost in both transmission and reflection operation modes. Convergent and retroreflective VCSEL sensors were also studied. A safety circuit was added to control the VCSEL output power within the Class I laser limit. Other potential VCSEL sensors such as a VCSEL light curtain device, dual-beam linear encoder, dynamic Moire optical torque sensor would also be discussed in the presentation.
VCSELs present peculiar features like the emission in several transverse modes and
polarization fluctuations. In some critical current regions, these lasers can emit
in two different states of polarization and/or transverse pattern for the same value
of the pump current. The laser dynamics in such narrow bistable regions is
characterized by noise-induced jumps between the two emission states. A
polarization and/or a spatial filter allows one to observe the random jumps as light
intensity hops. In this work, we evidence a novel phenomenon which is observed in
these conditions, that we have called Noise Assisted Binary Information Transmission
(NABIT): the addition of noise to the pump current up to an optimal value leads to a
strong improvement of the transmission quality, measured by the Bit Error Rate. These results
represent the first experimental evidence of Aperiodic Stochastic Resonance. We
present analytic calculations in good agreement with the measurements. We also
analyse the possible application to optical communications and compare it to a
standard amplitude modulation scheme.
Since VCSEL cavities posses a natural oscillation frequency, 1-D or 2-D periodic VCSEL arrays behave as oscillator lattices coupled through near neighbor interactions. Modulation of an
individual VCSEL will excite modulation waves through the lattice. A dispersion relation for the lattice waves is obtained using optical interaction among nearest VCSELs. It is shown that
the excited frequency bandwidth is given by the product of the VCSEL natural frequency times the coupling strength; the latter is parameterized by the ratios a/w and b/a, with a the VCSEL active radius, w the cavity mode waist and b the array period. A connection is made between the lattice wave decay constant
and the channel cross-talk for VCSEL array-based interconnects / switch fabrics.
We present a model to compute the linewidth in vertical-cavity surface-emitting
lasers, accounting for the 3D-structure of real devices. To this aim
we include the noise source in the field equations and treat both the noise
and the structural characteristics by means of coupled mode theory.
In this way we obtain an expression for the linewidth that is given as the
relation, modified by two correction factors that account for spatial effects
and modal dispersion of the resonator.
In the numerical results we give some guidelines towards devices aimed to
narrow-line emission and, as a significant example, we study the transition
from gain to index-guided devices in oxide-confined VCSEL's.