Vijay Jayaraman

Demonstration of broadband tunability in a semiconductor laser using sampled gratings

We propose and demonstrate a new monolithic laser structure in which the fractional tuning range Δλ/λ can be more than an order of magnitude larger than the fractional index change Δμ/μ in one section. The key idea involves using grating mirrors with grating elements removed in a periodic fashion. These ‘‘sampled gratings’’ have reflection spectra with periodic maxima. By using two such mirrors with identical grating pitch but mismatched sampling periods, it is possible to tune among the various reflectivity maxima. Our initial experimental results show 29.3 nm of tuning in an InGaAsP laser.

Extended tuning range in sampled grating DBR lasers

The authors demonstrate, for the first time, successful implementation of a distributed Bragg reflector laser with two sampled grating mirrors, which they previously predicted should give tuning range in excess of 50 nm. This device uses a uniform grating pitch throughout the structure, and relies on the same fabrication technology as a standard DBR laser. Initial results show 57 nm of mathematical tuning at 1.475 mu m with side-mode suppression ratio (SMSR)>30 dB over much of the tuning range. The observed change in operating wavelength versus mirror currents along with below threshold spectra agree with theoretical expectations.<<ETX>>

High-power 1320-nm wafer-bonded VCSELs with tunnel junctions

A new long-wavelength vertical-cavity surface-emitting laser structure is described that utilizes AlGaAs-GaAs mirrors bonded to AlInGaAs-InP quantum wells with an intracavity buried tunnel junction. This structure offers complete wavelength flexibility in the 1250-1650 nm fiber communication bands and reduces the high free-carrier losses and bonded junction voltage drops in previous devices. The intracavity contacts electrically bypass the bonded junctions to reduce threshold voltage. N-type current spreading layers and undoped AlGaAs mirrors minimize optical losses. This has enabled 134/spl deg/C maximum continuous-wave lasing temperature, 2-mW room-temperature continuous-wave single-mode power, and 1-mW single-mode power at 80/spl deg/C, in various devices in the 1310-1340 nm wavelength range.

Photoluminescence study of strain‐induced quantum well dots by wet‐etching technique

Simple holographic lithography and wet etching have been used to fabricate strain‐induced quantum well dot structures. Lateral confinement was generated in a GaAs quantum well (QW) by etching a double‐exposed grating pattern into a pseudomorphic, strained layer of In0.3Ga0.7As which overlies the QW. By spacing three QWs of different widths at varying depth from the stressor, lateral strain confinement and vertical strain propagation are directly resolved. We have observed at 14 meV redshift in the photoluminescence spectra for the QW located 22 nm away from the stressors and have confirmed that the strain propagation depth along the material growth direction is comparable to the lateral dot dimension.

Systematic observation of strain‐induced lateral quantum confinement in GaAs quantum well wires prepared by chemical dry etching

HCl radical beam etching has been used to produce strain‐induced lateral confinement in a GaAs quantum well. This confinement was generated in the GaAs quantum well by radical beam etching a grating pattern into a pseudomorphic, strained layer of In0.35Ga0.65As which overlies the GaAs quantum well. The photoluminescence spectrum showed two peaks, corresponding to the GaAs quantum well both beneath the strained layer and in regions where the strained overlayer had been etched away. The peak due to strain‐induced confinement displayed a redshift that increases with etch time; the maximum shift observed was 20 meV. The after‐etch photoluminescence intensity and the systematic peak shift with etch time are indicators of the degree of control and low‐damage nature of the etch process used.

2.5-Gb/s transmission over 50 km with a 1.3-μm vertical-cavity surface-emitting laser

We demonstrate error-free 50-km transmission at 2.5 Gb/s over standard single-mode fiber using an optically pumped 1.3-/spl mu/m vertical-cavity surface-emitting laser operating at 25/spl deg/C. No optical isolator or amplification was used in the transmission demonstration. System backreflection sensitivity measurements were conducted for various fiber distances and reflection feedback levels. The results indicate a transmission power penalty of 0.1 dB at a bit-error rate of 10/sup -9/ for a reflection feedback level of -25 dB with the reflection originating at 5 m.

Dynamic responses of widely tunable sampled grating DBR lasers

The modulation bandwidth, dynamic mode suppression ratio, and wavelength chirp of directly modulated sampled grating DBR lasers have been measured. Although the tuning range can be up to an order of magnitude larger than in simple DBR lasers, the chirp is about the same or better over a wide range of operation parameters. The modulation bandwidth was in excess of 4 GHz and the dynamic MSR remained larger than 40 dB as long as the current did not swing below threshold. The linewidth enhancement factor was extracted from the measured chirp parameters and ranged from three to eight for different lasing wavelengths of the tunable lasers. The dispersion of the linewidth enhancement factor is consistent with published theoretical predictions.

Continuous-wave operation of sampled grating tunable lasers with 10 mwatt output power, >60 nm tuning, and monotonic tuning characteristics

We describe recent results in widely tunable sampled grating lasers grown with tertiarybutylphosphine (TBP) and tertiarybutylarsine (TBA) column V sources. These devices exhibit 62 nm CW tuning range, 30-50 dB MSR, 10 mW output power, and monotonic tuning.<<ETX>>

Magnetotransport in lateral periodic potentials formed by surface‐layer‐induced modulation in InAs‐AlSb quantum wells

We report the realization of strong lateral superlattice potentials in not‐intentionally doped InAs‐AlSb quantum wells. By spatially alternating the surface layer between thin layers of InAs and GaSb, which shift the pinning position of the Fermi level at the surface, we induce a lateral density modulation of 4×1011 cm−2, equivalent to a potential modulation of 30 meV inside quantum wells 20 nm away from the surface. Both one‐ and two‐dimensional lateral potentials were fabricated and studied by magnetotransport measurements. Strong commensurability oscillations are observed.

Wafer-bonded VCSELs with tunnel junctions

We introduce a scheme incorporating wafer bonding and tunnel junctions to improve the performance long-wavelength Vertical Cavity Surface Emitting Lasers (VCSELs). Through careful design of PL-mode offset, mirror reflectivity, and aperture definition, we achieve lasing to 134°C, output power above 2 mW, single-mode output power at 80°C above 1 mW, and differential efficiencies of 46%. We achieve lasing at wavelengths as high as 1336 nm and show a versatile design that can be applied to any VCSEL functioning at long wavelengths.