Christopher W. Coldren

Incorporation of nitrogen in nitride-arsenides: Origin of improved luminescence efficiency after anneal

A key to the utilization of nitride-arsenides for long wavelength optoelectronic devices is obtaining low defect materials with long nonradiative lifetimes. Currently, these materials must be annealed to obtain device quality material. The likely defect responsible for the low luminescence efficiency is associated with excess nitrogen. Photoluminescence and capacitance–voltage measurements indicate the presence of a trap associated with excess nitrogen which decreases in concentration upon anneal. Our films are grown by elemental source molecular beam epitaxy and the background impurity concentration is low, thus we have investigated the role of crystalline defects. High resolution x-ray diffraction showed improved crystal quality after anneal. We observed that the lattice parameter does not decrease linearly with nitrogen concentration for levels of nitrogen above 2.9 mol % GaN. The fact that Vegard’s law is not observed, despite theoretical calculations that it should, indicates that nitrogen incorporat...

Widely tunable electroabsorption-modulated sampled-grating DBR laser transmitter

We report on a widely tunable transmitter based on a sampled-grating distributed Bragg reflector (SG-DBR) laser monolithically integrated with a semiconductor optical amplifier (SOA) and an electroabsorption (EA) modulator. Modulated time-averaged powers in excess of 5 dBm, RF extinction ratios >10 dB, and error-free transmission at 2.5 Gb/s for 350 km of standard single-mode fiber have been demonstrated across a 40-nm tuning range. In CW mode of operation, the module meets all long-haul system requirements for externally modulated laser sources: stability, power (>10 mW), RIN ( 100 yr for output wavelength stability and power across all channels.

High-speed, optically controlled surface-normal optical switch based on diffusive conduction

We report a surface-normal optically controlled optoelectronic modulator made from a reversed biased p-i (multiple quantum well)-n GaAs/AlGaAs structure with ultrathin barriers (5 A) whose recovery time is based on diffusive conduction. Modulation of reflectivity from 0.3 to 0.6 and back again in about 50 ps was demonstrated using a 750 fJ control pulse at 855 nm. We also demonstrated modulated changes in power greater than the control pulse power—a type of signal gain—by a factor of 1.8-to-1. Strong changes in reflectivity combined with low required control power make this device potentially useful for high-speed switching arrays in such applications as time division demultiplexing.

Low-threshold oxide-confined GaInNAs long wavelength vertical cavity lasers

We report, for the first time, room temperature continuous-wave (CW) operation of GaInNAs vertical-cavity surface-emitting laser diodes emitting at a wavelength of 1.2 /spl mu/m and grown all-epitaxially in a single step on a GaAs substrate. Oxide-apertured devices demonstrated CW threshold currents as low as 1 mA, slope efficiency above 0.045 W/A, and thermal impedance of 1.24 K/mW. Larger sized devices exhibited a pulsed threshold current density of 2-2.5 kA/cm/sup 2/ and slope efficiency above 0.09 W/A.

High performance widely-tunable SG-DBR lasers

Widely-tunable sampled-grating distributed Bragg reflector (SG-DBR) lasers with integrated Semiconductor Optical Amplifiers (SOAs) simultaneously exhibit high (20 mW CW) fiber-coupled output power, high side mode suppression ratio, low noise (below -140 dB/Hz RIN), low line-width (<5 MHz), and high reliability, across a 40 nm C-band tuning range.

Integration of active optical components

Integration of active optical components typically serves five goals: enhanced performance, smaller space, lower power dissipation, higher reliability, and lower cost. We are manufacturing widely tunable laser diodes with an integrated high speed electro absorption modulator for metro and all-optical switching applications. The monolithic integration combines the functions of high power laser light generation, wavelength tuning over the entire C-band, and high speed signal modulation in a single chip. The laser section of the chip contains two sampled grating DBRs with a gain and a phase section between them. The emission wavelength is tuned by current injection into the waveguide layers of the DBR and phase sections. The laser light passes through an integrated optical amplifier before reaching the modulator section on the chip. The amplifier boosts the cw output power of the laser and provides a convenient way of power leveling. The modulator is based on the Franz-Keldysh effect for a wide band of operation. The common waveguide through all sections minimizes optical coupling losses. The packaging of the monolithically integrated chip is much simpler compared to a discrete or hybrid solution using a laser chip, an SOA, and an external modulator. Since only one optical fiber coupling is required, the overall packaging cost of the transmitter module is largely reduced. Error free transmission at 2.5Gbit/s over 200km of standard single mode fiber is obtained with less than 1dB of dispersion penalty.

GaAs/AlGaAs multiple-quantum-well in-line fiber intensity modulator

We demonstrate a GaAs/AlGaAs multiple-quantum-well in-line fiber optic intensity modulator. Based on evanescent wave coupling between a GaAs/AlGaAs multiple-quantum-well waveguide and a single-mode fiber, this device concept combines the inherent advantages of in-line fiber devices with high-speed GaAs integrated optoelectronics. The GaAs waveguide uses distributed Bragg mirror layers to phase match to the low-index fiber. Intensity modulation of the transmitted light through the fiber is achieved by tuning the effective propagation index of the GaAs waveguide via the quantum-confined Stark effect. Initial structures show a modulation contrast (ΔT/T) of more than 53%, with an applied voltage of 5 V.

Group III nitride–arsenide long wavelength lasers grown by elemental source molecular beam epitaxy

Elemental source molecular beam epitaxy was used to grow InGaNAs quantum well samples, edge-emitting laser diodes, and vertical-cavity laser diodes on GaAs substrates. The quantum well samples exhibited an as-grown room temperature photoluminescence peak beyond 1310 nm which both increased dramatically in intensity and blueshifted with thermal annealing. Edge emitting laser diodes had threshold current densities as low as 450 and 750 A/cm2 for single and triple quantum well active regions, respectively, and emitted light at 1220–1250 nm. The vertical cavity laser diodes emitted light at 1200 nm and had threshold current densities of 3 kA/cm2 and efficiencies of 0.066 W/A.

Performance and reliability of widely tunable laser diodes

We are manufacturing widely tunable laser diodes based on an integrated single chip design. The sampled-grating distributed Bragg reflector (SG-DBR) laser is monolithically integrated with a semiconductor optical amplifier (SOA). The continuous wave (cw) output power of the fully packaged device is more than 2OmW over the entire C-band with low noise figures and a high side-mode suppression ratio of over 40dB. Devices are made for L-band operation as well. The devices are well suited for long-haul and ultra-long-haul DWDM optical transport systems. For metro type applications we developed an EML-type device with a SG-DBR widely tunable laser monolithically integrated with an SOA and an electro-absorption modulator (EAM) based on the Franz-Keldysh effect. The operating speed is 2.7GbiVs for OC-48 systems including forward error correction (FEC). The time averaged output power is in excess of 5dBm over the 4Onm C-hand tuning range. The extinction ratio is larger than IOdJ3:Error free transmission over more than 350km of standard single mode fiber is demonstrated. Wavelength switching is accomblished in less than lOms which is limited by the control electronics. The monolithically integrated widely tunable SG-DBR lasers show excellent reliability and environmental stability results. After testings hundreds of devices over several thousands of hours a long wear-out lifetime of over 350 years has been inferred with a very low failure rate of less than IOOFITs over the normal operating lifetime. The results indicate no reliability penalty for wavelength tunability compared to standard fixed wavelength DFB lasers.

Low threshold current continuous-wave GaInNAs/GaAs VCSELs

Vertical cavity surface-emitting lasers (VCSELs) emitting near 0.85 /spl mu/m are becoming increasingly important for short-haul optical fiber transmission systems. These devices benefit from highly reflective and thermally conductive all-epitaxial GaAs-based mirrors and efficient transverse confinement through AlAs-oxide dielectric apertures. Extending this commercially-established technology to wavelengths in the 1.3-1.6 /spl mu/m range allows for dramatically increased transmission bandwidth and distance in conventional single- and multi-mode fiber. GaInNAs is a promising active layer material grown on GaAs that can achieve 1.3 /spl mu/m emission, and electrically pulsed broad-area GaInNAs VCSELs have been realized. We demonstrate for the first time low-threshold (/spl sim/1 mA) GaInNAs VCSELs emitting at a wavelength of 1.2 /spl mu/m under continuous-wave room temperature operation.