Optically Pumped Semiconductor Lasers

Dr. Karen Ness

Optically pumped semiconductor lasers offer new solutions in telecommunications and could replace gas lasers in other markets.

The past few years have seen a resurgence of interest in optically pumped — rather than electrically pumped — semiconductor lasers. Optically pumped semiconductor lasers show significant promise as efficient, compact, reliable sources, offering power levels of up to several watts and good spatial mode quality at a range of wavelengths. Such performance, combined with developments in fabrication and device integration technologies, will lead to a class of versatile devices that could become the lasers of choice for a variety of applications in market sectors ranging from telecommunications to biomedical diagnostics.

The growing importance of these lasers as high-performance devices is due to their advantages in power scaling, spectral and temporal control, ease of design and processing, wavelength versatility and suitability for pumping with commercial pump diodes. They combine the design flexibility of a semiconductor-engineered gain region with extended — if compact — physical laser cavities that facilitate mode-locking or frequency conversion, thus extending the laser’s operating parameters.

Just emerging are tunable optically pumped semiconductor lasers, which provide a further degree of flexibility in performance. Tunability is enabled by microelectromechanical systems (MEMS) technology. Arguably, no other laser class offers engineers such design freedom, allowing them to tailor performance to specific application requirements.

Most development activity has focused on emission wavelengths around 1000 nm, with demonstrated continuous-wave (CW) power in a circularly symmetric TEM₀₀ mode now exceeding 1.5 W. These sources also have achieved stable single-frequency operation (unusually, from a linear cavity design) and more than 200 mW of average power in passively mode-locked picosecond operation.

In materials technology, the gallium arsenide family provides the basis for devices operating over significant portions of the visible and near-IR regions of the spectrum. GaAs-substrate-based epitaxial technology — using active regions including GaInP, GaAs, InGaAs and GaInNAs — in principle allows devices to be fabricated for any specific wavelength from 670 to 1500 nm. Effective pumping can be provided by commercially available pump diodes that emit at 650 to 670, 808 and 980 nm.

Intracavity doubling/tripling can generate output wavelengths down to the UV. The Coherent Sapphire laser based on this technology, with an output of 20 mW at 488 nm in a package slightly bigger than a computer mouse, is now available.

Based on the lasers’ flexibility, their applications are many — from specific, niche market lasers to larger, more “mass market” areas, often replacing less efficient or less reliable devices. In existing markets, they can challenge gas lasers in printing, biomedicine and scientific applications. As their performance reaches higher power levels, they could challenge crystalline host solid-state lasers in some industrial applications. They also could impact biological instrumentation, graphic arts, display, and inspection of silicon wafers and masks.

Telecommunications is one of the most exciting application areas for optically pumped semiconductor lasers, where — despite recent market downturns — the hunger for new technologies likely to underpin the recovery remains almost undiminished. Here the lasers can be viewed as a disruptive rather than a replacement technology, offering the potential for new solutions demanded by innovative network architectures. In recent years, the GaInNAs family (“dilute” nitrides) has been recognized for its potential for temperature-insensitive devices at wavelengths that are useful for telecommunications.

The move toward tunable lasers in the telecommunications industry also could involve the use of optically pumped lasers. It is estimated that 80 percent of all CW lasers will be tunable by the end of 2002, with the US market predicted to grow from about $5 million in 2000 to $1.2 billion by 2004. Some companies are already addressing this market by offering 1.55-μm MEMS-tunable optically pumped semiconductor sources.

These lasers could well become the lasers of choice for many markets, both current and yet to come.

Meet the author

Karen Ness is chief executive of the Institute of Photonics at the University of Strathclyde in Glasgow, UK.