Quantum-Dot Lasers’ Performance Makes Great Gains

Tom Tumolillo Jr.

The performance plateau of quantum-well technology has opened the door for its use in lowering costs and improving the performance of telecom and datacom applications.

The evolution of laser diode design has proved critical to the advancement of low-cost, high-performance transmitters for telecom and datacom networks, especially in light of the downturn in the optical communications market. With increased demand for lower-cost laser sources, companies are finding it impossible to meet these goals with current quantum-well technology, which has reached a performance plateau.

By comparison, quantum-dot technology offers a significant reduction in threshold current density (Figure 1). Other benefits include improved temperature performance, reduced chirp that enables 10-Gb/s direct modulation, increased lasing slope efficiency and less susceptibility to optical feedback. This allows isolator-free operation, an important step in reducing system costs. Reduced transmission laser costs translate directly to lower costs per bit for carrier network operators.

Figure 1. Besides a reduction in threshold current density, the evolution in laser diode design from double hetero to quantum-well and then quantum-dot technology also presents a corresponding improvement in the price/performance ratio (inset).

The current 1310- and 1550-nm quantum-dot laser diode technology is built upon existing quantum-well design. The quantum dots, formed in wafer deposition through molecular beam epitaxy, are semiconductor nanostructures that act as artificial atoms by confining electrons and holes in three dimensions. Working with either GaAs or InP substrates, the process forms initial deposition layers that are lattice-matched (or coherently strained) to the substrate, with deposition of the quantum-dot layer forming the active region. The InAs dots result from differences in the strain layers that cause self-assembly, also known as the Stransky-Krastanov process. The core material structure technology is called dots-in-a-well, or DWELL technology.

Completion of the laser structure involves depositing material layers lattice-matched to the substrate. Other than the quantum-dot layer, preceding and subsequent material layers are really no different from existing quantum-well laser structures. However, it is these tiny nanostructures that lead the device to a new standard of semiconductor laser performance (Figure 2).

Figure 2. The purer signal of quantum-dot technology compared with quantum-well designs will enable isolator-free operation and potentially eliminate up to 25 percent of the component cost.

Fueling the recent commercialization of quantum-dot systems are properties such as high temperature stability, a purer signal than conventional quantum-well devices and a broad gain spectrum. End users can expect further performance gains resulting from ongoing quantum-dot research by universities, consortiums and industry.

For example, the 1310-nm quantum-dot semiconductor optical amplifier has demonstrated an 18-dB gain, 9-dBm saturated output power, an 8-dB noise figure and a 10-ps gain recovery time. These results show the potential for operation free of pattern effects at high bit rates under saturation, and commercially viable gain and output power in a device that is roughly 10 times shorter in length than previous quantum-dot semiconductor optical amplifiers. A broad gain spectrum, temperature insensitivity and potential for an inherently lower noise figure will enable dot semiconductor optical amplifiers to challenge erbium-doped fiber amplifiers.

Theoretical work out of the Technical University of Denmark in Lyngby also shows the potential for quantum-dot semiconductor optical amplifiers to provide performance equivalent to that of erbium-doped fiber amplifiers, but at dramatically lower cost. Based on previous gain bandwidth data from the University of New Mexico in Albuquerque, the researchers predict that amplification could occur over a 200-nm range. They also want to take advantage of the ultralow linewidth of quantum-dot lasers — down to 100 kHz — for spectroscopic applications with wavelengths as long as 2 μm on InP substrates, as well as to investigate how dot density and multiple stacks of dots impact signal purity performance.

Further research efforts at the University of New Mexico are focusing on the development of 1310-nm quantum-dot vertical-cavity surface-emitting lasers for feedback insensitivity and high temperature stability. When it comes time to commercialize these devices, component designers should expect tests for reliability and lifetime to show that performance is equal to that of quantum-well devices. The mortality rate of Zia Laser’s product is comparable with that of existing InP and GaAs quantum-well systems, and engineers believe that quantum-dot technology should have no inherent reason for premature failure.

Anticipated innovations include direct-modulation 10-Gb/s 1310-nm uncooled distributed feedback performance with, eventually, isolator-free operation. Zia expects 10-Gb/s direct modulation to be the overwhelming solution for advanced optical components for at least the next decade based primarily on cost, performance and technological feasibility.

It is also important to consider that adoption of higher-speed networks takes many years to fully develop and implement. Along with OC-192 bit rates, uncooled performance is critical for lowering the cost of components. Because of the higher temperature-insensitivity properties of quantum-dot systems compared with quantum-well lasers, end users can expect to see future quantum-dot lasers reaching 100 °C performance.

Meet the author

Tom Tumolillo Jr. is director of product management at Zia Laser Inc. in Albuquerque, N.M.