Etched Mesa Buried Heterostructure Research Articles

16-channel tunable and 25-Gb/s EAM-integrated DBR-LD for WDM-based mobile front-haul networks.

We report a tunable distributed Bragg reflector-laser diode (DBR-LD) integrated with an electro-absorption-modulator (EAM) at an operating wavelength of 1.3 µm. This LD consists of gain, phase control (PC), DBR, and EAM sections, realized by using a butt-coupling technique in monolithically integrating the multiple quantum wells (MQWs) with the passive core and by applying an etched-mesa buried hetero-structure (EMBH) to the resonance cavity (i.e., gain to DBR section) and a deep-ridge type to the EAM section in fabricating the waveguide structure. Wavelength tuning of the LD is achieved by both applying a voltage to the heater metal of DBR section (coarse tuning) and injecting a current to the ohmic metal of PC section (fine tuning). From the work, the fabricated chips show a threshold current of about 13 mA, a side mode suppression ratio (SMSR) of more than 35 dB, and a tuning range of 15 nm within a heater voltage of 2 V. Dynamic tests for the EAM-integrated LD show the 3 dB bandwidth of more than 20 GHz and clear 25 Gb/s eye openings with a dynamic extinction ratio (DER) of over 7 dB for 16 channels spaced at the wavelength interval of 0.55 nm. Based on these results, we conclude that the EAM-integrated DBR-LD is capable of providing 16 channel operation at a data rate of 25 Gb/s and can be used as an effective light source for WDM-based mobile front-haul networks.

Multi‐wavelength channel 1.5 μm and 10 Gb/s EMBH DFB‐LD array module using SAG technology

A multi-wavelength channel distributed feedback laser diode array (DFB-LDA) for the low-cost and low-power consumption applications of WDM-based datacentre networks is reported. The gain spectrum and lasing wavelength of each channel are controlled by using selective area growth and E-beam lithography techniques, respectively. To reduce the operation current and parasitic capacitance of the proposed array, an etched mesa buried hetero-structure (EMBH) is introduced. From this study, it was found that an eight-channel 300 μm long λ/4-shifted DFB-LDA has a threshold current of 10 ± 1.5 mA and a side mode suppression ratio of over 50 dB at a current of 50 mA for all channels. A transmission test for this LDA showed a clear eye opening with a dynamic extinction ratio of over 4.5 dB at 10 Gb/s after a 2 km transmission.

1.5-μm and 10-Gb s−1 etched mesa buried heterostructure DFB-LD for datacenter networks

We report a 1.5 μm and 10 Gb s−1 etched mesa buried heterostructure λ/4-shifted distributed feedback laser diode (DFB-LD) for the low-cost application of WDM–based datacenter networks. To reduce the threshold current and improve the modulation bandwidth in a conventional p-/n-/p-InP current blocking structure, a thin undoped-InP (u-InP) layer was inserted between the side walls of the active region and the p-InP layer (i.e., a u-/p-/n-/p-InP structure), and the region containing the active region and the current blocking structures was etched in a mesa form (i.e., an etched mesa). From this work, it was found that a 300 μm long anti-reflection (AR)-AR DFB-LD with a mesa width of 8 μm is reduced by about 25% while a side mode suppression ratio is >50 dB and a 3 dB bandwidth is >10 GHz at a current of 40 mA; in addition, it shows a clear eye-opening with a dynamic extinction ratio of >4.5 dB at 10 Gb s−1, and a power penalty of <1 dB after a 2 km transmission.

The fabrication and performance of 1.3μm DFB EMBH lasers with coplanar contacts

Abstract The fabrication and performance characteristics of coplanar contact etched mesa-buried heterostructure (EMBH) distributed feedback (DFB) lasers emitting at 1.3μm wavelength are described. The processing was designed such that the lasers could be evaluated as coplanar contact or conventional (top/bottom) contact devices. The threshold current was as low as 14mA and the 3dB small signal response was as high as 9.4 GHz. Both these properties showed negligible differences when the device was biased either coplanarly or conventionally.

Tungsten metallization for stable and self-aligned InP-based laser devices

Tungsten thin films on n-InP layers have been investigated for potential use as a refractory ohmic contact for self-aligned In-based etched mesa buried heterostructure laser devices. The W films were rf sputter deposited onto InP substrates, S doped in the range of 1×1018–1×1019 cm−3. The deposition parameters were optimized to produce films with the lowest possible induced stress, minimum argon content, and best morphology for as-deposited wafers and after undergoing reactive-ion etching and high-temperature thermal cycles (700 °C), which are required for the self-aligned technology. These parameters were obtained for films that were rf sputter deposited at a discharge power of 240 W and under argon pressure of about 10 mTorr. A thermal expansion coefficient of 5.84×10−6 C−1 and a biaxial elastic modulus of 0.97×1012 Pa were measured for the films. The electrical behavior of the W films sputtered onto n-InP was studied by means of I-V and contact resistance measurements, which revealed a linear ohmic contact as-deposited, while sputtered onto n-InP substrates, S doped to the level of 5×1018 cm−3 or higher. Rapid thermal processing at elevated temperatures improved the ohmic contact quality and decreased the specific contact resistance values to a minimum of 3.5×10−6 Ω cm2 as a result of heating the W/InP (S doped 1×1019 cm−3) at 600 °C.

Defect Mechanisms in Degradation of Long Wavelength (1.30–1.55 Micron) Laser Diodes

ABSTRACTOptical degradation of long wavelength (1.30–1.55 micron) laser diodes during normal operation or accelerated aging test caused by lattice structural deterioration of the active region materials and mirror facet damage were investigated in detail. Extrinsic dislocation loops of 1/2&lt;100&gt;{010) types were observed in gradually degraded channeled-substrate-buried-heterostructure (CSBH) lasers. These dislocation loops, originated at the sidewall interfaces outside the active region, grew into the active region in the direction of minority carrier injection. A great enhancement of the loops' growth rate was observed after they entered the active region, indicating a nonradiative recombination enhanced defect reaction under the strong optical field. Furthermore, the &lt;100&gt; oriented extrinsic dislocation loops were confirmed to be dark-line-defects (DLDs). On the other hand, strong nonradiative recombination centers, created by mirror facet damage, or pre-existed internally inside the cavity, resulted localized melting on the {111} planes. The propagation of the localized melt-patch along the laser beam direction created a wormlike defect along the laser cavity, which degraded the laser device catastrophically. Various types of grown-in defects for CSBH and etched-mesa-buried-heterostructure (EMBH) laser devices are described and their effects on the device performance are demonstrated.

Observation of anomalous far-field intensity distributions in semiconductor lasers and their explanation

The observed variations in the far-field intensity distributions of index-guided lasers correlates well with variations in the near-field intensity distributions. The far- and near-field nonuniformities (large intensity variations generally attributed to scattering) arise from variations in the active region dimensions along the length of the optical cavity. Model calculations are presented which show that 20% variation in active width can cause 20%–40% intensity modulation in the far field. The magnitude of the observed far-field nonuniformities depends on the laser structure and the details of the fabrication process. For example, the capped mesa buried heterostructure lasers generally show less scattering-type far-field nonuniformities than the etched mesa buried heterostructure lasers.

Dynamic and CW linewidth measurements of buried heterostructure distributed feedback lasers

Measurements of frequency chirp as a function of modulation current, data rate, and bias level for the etched-mesa buried heterostructure distributed feedback (EMBH-DFB) laser are presented. The results show that the chirp increases with increasing modulation current and is significantly larger if the laser off-state is below threshold than if it is above-threshold. The 20-dB down chirp widths are in the range of 4 to 6 AA for 40 mA of modulation current at 5 Gb//s under above threshold bias. Using the EMBH-DFB laser, a system experiment over 22-km fiber at 16 Gb/s has been carried out which shows less than 0.5 dB dispersion power penalty. The continuous-wave (CW) linewidths of the asymmetric facet-coated DFB lasers are in the range of 10 to 40 MHz at 6 mW of output power. This wide range arises principally from a variation of the phase of the grating at the high-reflectivity coated facet. >

GaInAsP dual wavelength laser

The fabrication and performance characteristics of a dual wavelength laser emitting near 1.3 mu m and 1.55 mu m are described. The lasers are of the etched mesa buried heterostructure type, and utilise semi-insulating InP layers both the lateral optical confinement and current confinement. The 1.55 mu m emission is at a single wavelength by virtue of the frequency selective feedback provided by a grating etched on the substrate. The lasers have threshold currents in the 20 to 40 mA range, and have quantum efficiencies comparable to single emitter lasers.

Reproducible liquid phase epitaxial growth of InGaAsP buried heterostructure lasers

Buried heterostructure lasers are formed using double heterostructure planar layers which are masked and etched to define laser mesas, with final regrowth by liquid phase epitaxy. Controlled melt etching of the exposed wafer surface is introduced just prior to the liquid phase epitaxial regrowth to form etched mesa buried heterostructure lasers. The melt etching uses an In-InP melt undersaturated by only ∼0.2 °C and does not degrade the dimensional shape of the etched mesas. This process is shown to dramatically improve the growth reproducibility and results in excellent device characteristics. This closely controlled removal of the mesa sidewall just prior to epitaxy is also expected to contribute to increased laser reliability.

Performance Comparison of InGaAsP Lasers Emitting at 1.3 and 1.55 μm for Lightwave System Applications

Experimental results relative to the performances of real index-guided InGaAsP lasers emitting near 1.3 and 1.55 μm are described and compared. The laser structures discussed are the etched mesa buried heterostructure, channeled substrate buried heterostructure, and the double channel planar buried heterostructure. The effect of Auger recombination and intervalence band absorption on the threshold current and external differential quantum efficiency is discussed. The effect of the larger Auger coefficient at 1.55 μm is compensated by a lower carrier density at threshold at 1.55 μm so that the total nonradiative current loss for lasers emitting at 1.55 μm is not significantly larger than that for lasers emitting at 1.3 μm. A small linear shunt leakage current (∼10 mA) can increase the T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf> to ∼100K. We report threshold currents as low as 11 and 15 mA (at 30°C) and continuous-wave operating temperatures as high as 130 and 110°C for lasers emitting at 1.3 and 1.55 μm, respectively.

Effect of active layer placement on the threshold current of 1.3-μm InGaAsP etched mesa buried heterostructure lasers

The threshold current of InGaAsP etched mesa buried heterostructure (EMBH) lasers is strongly influenced by the position of the active layer in the etched mesa. Transmission electron microscopy results are presented which show the presence of a thin planar disorder along the etched 111A interface. Nonradiative recombination of carriers is believed to be responsible for the increased threshold current of EMBH lasers whose active region is bounded by 111A interface. Lasers with threshold current as low as 14 mA at 30 °C have been fabricated by proper placement of the active layer and by optimizing layer thicknesses and doping levels.

Analysis of leakage currents in 1.3-µm InGaAsP real-index-guided lasers

We describe leakage current calculation in several real-index-guided laser structures using an electrical equivalent circuit model. The structures analyzed are different types of buried heterostructures in which heterojunctions are used for lateral carrier confinements. The device types are: 1) the etched-mesa buried heterostructure (EMBH); 2) the channeled-substrate buried heterostructure (CSBH); 3) the double-channel planar buried heterostructure (DCPBH); 4) the planar buried heterostructure (PBH); and 5) the buried crescent (BC). Adequate current confinement is necessary in a laser structure for both linearity and low threshold operation. Thus leakage current, i.e., the difference between the injected current and the current through the active region, should be small. We have identified the main leakage paths in these structures and the parameters (the relevant layer thicknesses and doping levels) that determine the magnitude of the leakage current. The effect of nonradiative recombination sites on junction parameters and the consequent increase in leakage current is discussed.

High-frequency characteristics of directly modulated InGaAsP ridge waveguide and buried heterostructure lasers

The high-frequency modulation characteristics of InGaAsP ridge waveguide lasers at 1.55 μm and etched mesa buried heterostructure (EMBH) lasers at 1.3 μm are investigated. Small-signal and large-signal circuit models are developed for both devices, and the main factors which influence the high-frequency modulation response are established. It is shown that the electrical parasitics in the chip dominate the small-signal frequency response of the EMBH laser and limit the large-signal turn-on and turn-off times. The small-signal and large-signal responses of both devices show strong damping of the relaxation oscillations. This damping can be modeled accurately using field-dependent optical gain compression in the rate equations.