A. Von Lehmen

Dynamic, polarization, and transverse mode characteristics of vertical cavity surface emitting lasers

The dynamic, polarization, and transverse mode characteristics of strained InGaAs-GaAs quantum well vertical cavity surface emitting lasers (VCSELs) emitting at 0.98 mu m are investigated. The dynamic behavior of VCSELs with high and low operating voltages and series resistances is compared. A large wavelength chirp in the lasing spectrum was observed for the lasers with high voltage/resistance, even under low-duty-cycle pulse operation. This is thought to be due to resistive heating close to the laser junction. It is observed that the transverse mode structure of VCSELs and their dependence on laser dimensions and drive current are highly analogous to those of edge emitting lasers, whereas the polarization characteristics of the two types of lasers are significantly different. >

Optical Stark effect on excitons in GaAs quantum wells

We describe the first reported experimental observation of an extremely fast shift of the n = 1 exciton transition energy in GaAs quantum-well heterostructures. The shift is produced by optical pumping below the band gap and is not associated with a carrier or exciton population. We interpret the shift in terms of an optical Stark effect. We present a model for the Stark effect on the ground-state exciton in quantum wells and find good agreement between the predictions of the model and our experimental results.

Transverse mode characteristics of vertical cavity surface-emitting lasers

Transverse mode characteristics of vertical cavity surface‐emitting (VC‐SE) lasers are described. The mode structure is investigated as a function of the transverse dimension for proton‐implanted gain‐guided VC‐SE lasers. A comparison is made to an air‐post index‐guided structure. The lasing modes and the evolution of the modes with increasing drive current for the VC‐SE lasers are observed to be highly analogous to those of the edge‐emitting lasers. Broad‐area gain‐guided lasers lase in the fundamental TEM00 mode near threshold. At higher currents, high‐order modes are successively excited. A 5 μm square proton‐implanted gain‐guided VC‐SE laser emits a single mode. On the other hand, an air‐post index‐guided SE laser, due to the large index difference between the laser and the cladding, emits multiple transverse modes. Moreover, we show that the gain‐guided VC‐SE lasers exhibit better device characteristics than the air‐post index‐guided lasers.

Network Design and Architectures for Highly Dynamic Next-Generation IP-Over-Optical Long Distance Networks

The DARPA CORONET project seeks to develop the target network architectures and technologies needed to build next-generation long-distance IP-over-Optical-Layer (IP/OL) networks. These next-generation networks are expected to scale 10-100 times larger than todays largest commercial IP/OL network. Furthermore, DARPA has established advanced objectives for very rapid provisioning of new IP or private line connections, very rapid restoration against up to three simultaneous network failures, and future dynamic ldquowavelengthrdquo services ranging from speeds of 40-800 Gigabits per second. Besides these ambitious goals, the CORONET project seeks to establish a commercially-viable network architecture that supports both commercial and government services. In this paper, we describe the CORONET program requirements, and present our initial architectures and analysis of the early phases of this long-term project. We propose a novel 2-Phase Fast Reroute restoration method that achieves 50-100 ms restoration in the IP-Layer in a cost-effective manner, and a commercially viable OL restoration method that can meet the rapid CORONET requirements. We also estimate the magnitude of the extra capacity needed to provide dynamic wavelength services compared to that of static services, and show that the extra capacity to restore a small percentage of high priority traffic against multiple failures requires a small amount of extra capacity compared to that of single failures.

Fabrication of a Two-Dimensional Phased Array of Vertical-Cavity Surface-Emitting Lasers

We report the successful fabrication of a two‐dimensional phase‐locked array of vertical‐cavity surface‐emitting lasers. The array was comprised of more than 160 vertical‐cavity surface‐emitting lasers of 1.3 μm diameter with a separation of less than 0.1 μm between each lasing element. The array had a 25 μm diameter and each of the elemental lasers was located on a two‐dimensional rectangular lattice. The threshold current of the two‐dimensional array 45 mA yields a threshold current of 280 μA for an elemental laser. The far‐field beam angle of the array was as narrow as 7°, and the spectral purity was found to be good enough to allow for a clear holographic image reconstruction of a holographic memory.

Vertical-cavity surface-emitting InGaAs/GaAs lasers with planar lateral definition

A planarity preserving method for the definition of vertical‐cavity surface‐emitting lasers (VC‐SEL) is described. A strained‐layer InGaAs quantum well VC‐SEL structure was grown and lasers were laterally defined using a tailored deep proton implantation process. In these lasers we obtained low threshold current densities of 1000 A/cm2 and efficient cw operation. This method facilitates large‐scale integration of VC‐SEL devices.

Optically controlled surface‐emitting lasers

Vertical cavity surface‐emitting lasers are integrated with GaAs/AlGaAs heterojunction phototransistors to yield optically controlled lasers. The two‐terminal device operates in several modes: As an amplifier with a large signal, external optical gain of 5, as in optical logic gate, and as an optically or electrically triggerable latch. The optical AND gate has an output on‐to‐off ratio of 10:1. Although the device has no optical feedback, latching is achieved with appropriate biasing through impact ionization. The device structure is advantageous for forming large two‐dimensional arrays for optical signal processing.

Integration of planar Fresnel microlenses with vertical-cavity surface-emitting laser arrays

We report a technique of integrating planar Fresnel microlenses with InGaAs/GaAs-based vertical-cavity surfacee-mitting laser arrays by selectively ion-beam milling the substrate. Depending on the application, one can focus, collimate, and bend the individual laser beams using such microlenses. An example is presented where a 32 x 32 array of microlenses, each with an aperture of 80microm and a focal length of 108 microm is integrated with a laser array. As expected, arrays of focused beams, each with a 2-microm spot size, are generated at a distance of approximately 110 microm.

VCSE laser transmitters for parallel data links

The major issues associated with using vertical cavity surface emitting laser (VCSEL)-based transmitter arrays for parallel data links are examined. An assumption is made that only limited optimization of the individual links is permitted, and to the extent that the required circuits are shared by many channels. An analysis of bit-error ratios, based on measured laser parameter variations across the array, and their theoretical effect on noise margins indicates that a 5-dB penalty may be expected. This leads to a tradeoff between laser tolerances and receiver complexity. Experimental work towards a VCSEL-CMOS transmitter is described. A 622-Mb/s operation is demonstrated with a small hybrid transmitter array. Power dissipation and electrical crosstalk problems are assessed. Block coding of data before transmission significantly improves some aspects of laser transmitter performance. A statistical treatment of crosstalk noise is presented and compared to experimental results. >

Two-wavelength absorption modulation spectroscopy of bandtail absorption in GaAs quantum wells

We have discovered that below‐band‐gap photoexcitation produces large bleaching of the exciton absorption in GaAs quantum well heterostructures. We have used this effect to perform the first investigation of room‐temperature bandtail absorption in these structures. We find that the below‐band‐gap absorption follows a spectral Urbach’s rule. In addition, proton‐bombarded samples show an Urbach energy correlated with bombardment‐induced defects. This sensitive technique has enabled us to study samples as thin as 1 μm at energies where the absorption coefficient is ∼10 cm−1.