High density wavelength division multiplexing (DWDM) requires several channels with a fixed wavelength spacing of 1 - 2 nm. This can be achieved by fabricating arrays of distributed feedback (DFB) lasers, using a photomask method to print the gratings with a different period for each DFB laser in the array. The grating phase mask is fabricated first, using e-beam lithography and reactive ion etching. The parallel exposure of all gratings using such a mask is orders of magnitude faster than direct wafer writing, using e-beam lithography. The characteristics of 8-channel gain coupled DFB laser arrays designed for a 2 nm channel spacing are reported. The required performance, achievable yield (i.e. cost), packaging requirements and reliability will determine whether these arrays are suitable for use in DWDM systems.
Aluminum-free material has proved to be very promising for lasers of 800 - 1000 nm wavelength range. Up to now the most widely used growth method of GaInAsP quaternary alloys was Metal-Organic Chemical Vapor Deposition (MOCVD) technique. Gas Source Molecular Beam Epitaxy (GSMBE) is also able to produce high-quality Al-free material for optoelectronics. This paper aims to present the direct comparison of laser material quality grown by MOCVD and GSMBE. The easiness of composition control, flexibility of the deposition process and composition uniformity in GSMBE-grown material allowed us to further improve the performance of laser diodes operating at 800 nm wavelength range.
The performance characteristics of InGaAs/GaAsP/InGaP strain compensated laser emitting near 1.0 micrometers are reported. The ridge waveguide lasers have room temperature threshold current of 16 mA and differential quantum efficiency of 0.45 W/A/facet. Lasers with far field divergence of 17 degree(s) X 15 degree(s) have been fabricated. A small signal bandwidth of 38 GHz has been obtained using a strain compensated structure.
Despite their complexity, vertical cavity surface emitting lasers (VCSELs) have become key devices for future low cost optical interconnections. The high quality dielectric distributed Bragg reflectors (DBRs) mirrors possible in the AlGaAs system have made GaAs based devices the most successful and most studied VCSELs. This paper reviews the work carried out at the University of Sheffield on devices which employ AlGaAs mirrors and emit at wavelengths across the range 640 to 1100 nm. The active layers in the different designs contain variously quantum wells of InGaAs, GaAs, AlGaAs, and AlGaInP. A major limitation of using Al based compounds, particularly AlGaAs, at shorter wavelengths has previously been the presence of oxygen and other impurities. But by improving the crystal quality and purity, respectable performance of arsenide compounds has been extended to the sub 700 nm wavelength region and further improvements are expected through structural optimization and the application of strained AlInGaAs layers. Issues regarding the growth, device resistance and reproducibility of emission wavelength are also discussed.
By utilizing the gain compression mechanism in a semiconductor optical amplifier, a wavelength conversion system with distribution to two new wavelengths has been demonstrated. The modulation depths of the output wavelengths were found to vary inversely with the probe input powers. Results of a wavelength conversion and distribution system used to transfer subcarrier multiplexed data to two new wavelengths is presented.
A grating coupled surface emitting semiconductor laser consists of an in-plane laser with an integrated grating coupler that couples the light from the laser out of the waveguide and simultaneously shapes the emitted beam. Here we report on the progress towards a fully integrated grating coupled surface emitter. This includes: (1) grating outcouplers exhibiting more than 90% outcoupling efficiency through the use of optimized grating profiles and waveguide structures, (2) grating outcouplers possessing beam shaping capabilities through the use of computer generated waveguide holograms, and (3) grating based in-plane lasers producing wide and spatially coherent guided modes through the use of an unstable resonator design and mode selective feedback elements.
The understanding of failure mechanisms in InGaAs/AlGaAs 980 nm pump lasers has gained significantly in the last couple of years. Random failure reduction has been observed as a result of improved facet-passivation techniques, and improved packaging and chip technology. With todays wear-out extrapolation data and random-failure activation estimates, there is clearly outlook for operation at higher output- power levels (200 - 250 mW) than used so far at reasonably low random-failure rates and low failure-rate operation at lower power levels (e.g. 60 mW, 25 degree(s)C, towards 25 FIT?), which clearly stimulates the interest for application of 980 nm pumps in submarine applications.
Laser diodes packaged in receptacles are attractive candidates for the deployment of optics in the Access Network because of their simple design and easy handling. Nevertheless, such devices may suffer from specific issues which affect their output power stability. Two different types of receptacle technologies based respectively on the FC/PC and the EC connector concepts have been carefully investigated. The electro-optic characterization has mainly consisted in the recording of P(I) over a large temperature range and in the measurement of mating repeatability. In addition, the inner `optical structure' of each type of device has been investigated by low coherence reflectometry. The results obtained show that the EC concept makes it possible to design laser diode receptacles with good properties: repeatability (approximately equals 0.1 dB) and random mating (approximately equals 0.5 dB). On the other hand, FC/PC receptacles exhibit random mating higher than 1 dB, a fact which illustrates the strong impact of the patchcord on coupled power. In addition, depending upon the patchcord and temperature of testing, P(I) curves of FC/PC receptacles may exhibit strong non-linearities. The investigation of such devices with low coherence reflectometry shows that they are due to multiple optical reflections in the cavity formed by the front facet laser and the fiber end. Such a behavior highlights the weakness of the basic concept of FC/PC receptacles. That physical contact is difficult to maintain whatever the patchcord and the environmental temperature conditions.
GaInAsP buried heterostructure pump lasers emitting at approximately 1480 nm have been subjected to simulated electrostatic discharge pulses, with one added feature over usual practice in that the lasers were run at close to maximum output while the pulses were applied. Using pulses in the forward bias direction, no perceptible damage occurred at up to 50 kV from a standard human body model of 100 pF discharging through 1500 (Omega) and the device under test. ESD pulses in the reverse direction caused damage at 5 to 12 kV, and the damage threshold was increased by about 600 V when the lasers were running during the test. Analysis of data, and physical arguments reveal the likely nature of damage and its causes, and the applicability of this theory to other device structures and wavelengths is discussed.
In this paper, we focus on how vertical-cavity surface- emitting lasers (VCSELs) and arrays have led to many feasible advanced technological applications. Their intrinsic characteristics, performance, and producibility offer substantial advantages over alternative sources. Demonstrated performance of `commercial-grade' VCSELs include low operating powers (< 2 V, mAs), high speeds (3 dB BWs > 15 GHz), and high temperature operating ranges (10 K to 400 K and -55 degree(s)C to 125 degree(s)C, and T > 200 degree(s)C). Moreover, their robustness is manifest by high reliability in excess of 107 hours mean time between failures at room temperature and tenfold improvement over existing rad-hard LEDs. Hence, even these `commercial-grade' VCSELs offer potential within cryogenic and avionics/military or space environments. We have also demonstrated submilliamp ITH, stable, single-mode VCSELs utilized within bias-free 1-Gbit/s data links. These low- power VCSELs may also serve in applications from printers to low-cost atomic clocks. The greatest near-term VCSEL applications are upgrades to low-cost LEDs and high-grade copper wire in data links and sensors. Exploiting their surface-emitting geometry, VCSELs are also compatible with established multichip module packaging. Hence VCSELs and VCSEL arrays are ideal components for interconnect-intensive processing applications between and within computing systems.
Controlled lifetests totaling 1.9 million device hours have been carried out on a distributed feedback (DFB) laser emitting at a wavelength near 1.55 micrometers with an output power in excess of 50 mW. The degradation rate is typically found to decrease with increasing operating time for a wide range of drive currents and temperatures. The effective activation energy was found to be 0.52 eV. End of life was defined by the following three criteria: the maximum operating power decreasing to 48 mW; the change in the center wavelength due to aging equaling +/- 0.5 nm; the operating current increasing by 20% (approximately equals 40 mA). Conservative predictions of operating lifetime are made using the worst case of constant degradation over the lifetime and an activation energy of 0.4 eV. Less than 0.01% of the sample are expected to wear-out in 20 years at 30 degree(s)C. No random failures were observed giving a random failure rate below 32 FITs. These results provide a high degree of confidence that the 1550 nm MQW DFB high power laser structure investigated is fit for both high speed optical communications and CATV systems.
Our studies of device lifetime and the main degradation mechanisms in Nichia blue LEDs date back to Spring 1994. Following the initial studies of rapid failures under high current electrical pulses, where metal migration was identified as the cause of degradation, we have placed a number of Nichia NLPB-500 LEDs on a series of life tests. The first test ran for 1000 hours under normal operating conditions (20 mA at 23 degree(s)C). As no noticeable degradation was observed, the second room temperature test was performed with the same devices but with a range of currents between 20 and 70 mA. After 1600 hours, some degradation in output intensity was observed in devices driven at 60 and 70 mA, but it was still less than 20%. The subsequent tests included stepping up the temperature by 10 degree(s)C in 500 h intervals up to a final temperature of 85 degree(s)C using the same currents applied in the second test. This work reviews the failure analysis that was performed on the degraded devices and the degradation mechanisms that were identified.
High performance, low cost, and highly reliable vertical cavity surface emitting lasers (VCSELs) have been developed and are currently being used in both parallel and serial optical interconnect applications. For example, Motorola's OPTOBUSTM parallel optical interconnect relies heavily on the unique characteristics of arrays of GaAs based VCSELs emitting at 850 nm to achieve its stringent performance goals at low cost. Representative parametric results of discrete VCSELs and VCSEL arrays will be compared, including `optical power output-current' and `current-voltage' curves, optical wall plug efficiencies, and modulation characteristics. The use of statistical parameter analysis across a wafer and subsequent parametric wafer maps has proven to be a valuable tool for maintaining control of the fabrication process. The consistency of VCSEL parameters across individual VCSEL arrays will be discussed. VCSELs are very robust devices. Life times at room ambient in excess of 3E6 hours have been reported by several groups. Degradation behavior of selected device parameters will be discussed. Failure analysis demonstrating the effect of proton implant depth on reliability will be presented. ESD damage at forward bias is shown to be process related, while ESD damage at reverse bias is shown to be material related. These VCSELs are ESD Class 1 devices.
The well-known method of photocurrent spectroscopy, i.e. the measurement of the spectral sensitivity of a laser diode like for a detector, was found to monitor aging properties of (In)AlGaAs/GaAs high power laser diode arrays (LDA) in a convenient way. Photocurrent spectra of LDAs emitting at 808 nm (1.53 eV) were measured in the 0.8 - 3.0 eV photon energy range. Aging induced changes in different spectral regions reveal the influence of different mechanisms affecting the structure. Conclusions on the microscopic nature of the changes are drawn and applications are discussed.
We have performed a structural study of GaAlAs/GaInAs lasers, designed to emit at 980 nm, ass they undergo catastrophic optical damage (COD) during L-I testing. Electron beam induced current analysis shows that COD is characterized by the introduction of high densities of extended defects along the cavity of the laser, in the vicinity of the output facet of the device. Having determined the approximate location of the induced defects, we have employed precision transmission electron microscope specimen fabricating techniques to study the structure of the defects in detail. Dislocation loops ranging from < 10 nm to > 100 nm in diameter are observed in the modal region of the degraded lasers. The heterostructure comprising the active region of the device is interdiffused in regions of the highest defect density, and in some regions, melting of the laser cavity is observed. A `fast capture' laser degradation analysis technique has been developed, which allows us to examine the infant stages of COD failure. These experiments clearly demonstrate that the COD damage initiates exactly at the laser facet, and propagates back along the cavity with continued device stressing. The highest output power level which can be attained is limited by the thermal characteristics of the device. Higher power levels can be attained under pulsed operation, through which `thermal rollover' conditions are avoided. COD failure under pulsed operation results in a dramatically altered defect distribution consisting of periodic arrays of dislocation tangles along the laser cavity. The periodicity of these defective `packets' is related to the magnitude of the drive current pulse at the time of failure.
The high speed characteristics of Vertical Cavity Surface Emitting Lasers (VCSELs) for use in modern high bandwidth fiber optical networks is presented. An equivalent circuit model based on microwave network analyzer S11 measurements is developed. The dynamic operation of multi- transverse mode VCSELs is also investigated. Experimentally, a laser with two orthogonally polarized modes is examined. We show that each of the transverse laser modes may have significantly different rise and fall times. A multimode rate equation model is used to predict the exact pulseshape for each mode. The laser gain is saturated by the total optical intensity, and the sum of the modal powers is shown to have a constant rise and fall time. The system performance in terms of the bit error rate is also investigated. We demonstrate that selective attenuation of the optical modes can lead to an increase in the bit error rate due to polarization partitioning noise.
Measurements of longitudinal variation in critical laser parameters such as gain and carrier concentration are invaluable in understanding and diagnosing device performance and failure mechanisms. However, traditional front-facet measurements cannot reveal the variation of these parameters along the length of the laser. Other methods require physical modifications to the laser itself, such as the fabrication of a top window, and are thus invasive. We describe a new experimental technique based on analysis of side spontaneous emission. A tapered optical fiber translated along the side of the laser using a micropositioner collects spontaneous emission from the active region, allowing spatially-resolved gain and carrier concentration measurements to be made. Such measurements can be used to track the evolution of dark lines caused by defects where non-radiative recombination is dominant. We applied this method to a 980 nm high power laser with an In0.2Ga0.8As, 80 angstroms SQW and facets of 90%/10% reflectivity. It was predicted through a 1D rate equation model that the carrier concentration would increase near the high-reflectivity mirror, due to lower optical field intensities. Using the bimolecular recombination equation to determine the carrier density, this expectation was confirmed. The peak modal gain also increased with proximity to the high-reflectivity mirror, and modulations in the gain peak profile attributed to spatial hole burning were observed.
The use of optical devices in telecommunications in the access arena requires cost and size reduction, product simplification and high stability. These have been achieved by the use of a silicon optical bench. The technology allows passive alignment to the laser diode by locating the fiber in a Vee groove precision etched in a silicon substrate. An additional advantage of this technology is the inherent robustness of the coupling of the fiber to the laser diode emission which has enabled both high reliability and stability to be achieved. This paper presents the main features of the technology, including the controls required during manufacture, and the resulting high stability of the device over extremes of environmental conditions.