Conventional lasers are very complex systems that can also be inefficient.This is because they can only emit light in a discrete set of available wavelengths: and not necessarily the best for the intended applications. On the other hand, semiconductor diode lasers, while wavelength-flexible and extremely practical as light sources, are also limited when it comes to the optical characteristics of their output beams.
Many applications require visible laser radiation. In past decades, this could only be obtained using gas lasers, which are notoriously bulky and inefficient. With the development of diode-laser pumps, solid-state laser technology has now gained wide acceptance in spite of the limited choice of colors it provides. We recently developed a novel type of solid-state laser using semiconductor materials, but not in the form of a diode. Our new optically pumped semiconductor (OPS)1 technology allows us to get the best of both worlds: the superb laser properties of semiconductor materials combined with the design flexibility of solid-state lasers.
To take full advantage, our approach is to use these solid-state laser materials as we would any other, pumping them optically and building them into conventional laser cavities (see Figure 1). However, by not restricting ourselves to a monolithic implementation, we can use discrete optical components and free space propagation to modulate the characteristics of the output beam. The ability to access the intracavity radiation also allows us to include a frequency doubling crystal to generate visible wavelengths.
Figure 1. Schematic representation of an optically pumped semiconductor laser, including the chip design.
The OPS chip consists of two sections: the gain medium at the top and the high-reflecting mirror at the bottom. The gain at the laser wavelength is provided by narrow layers of lower bandgap materials (quantum wells). The pump is filtered out by using spacer layers that are transparent to the laser wavelength and strongly absorbent to the pump. Our interest in OPS technology was initially sparked by the possibility of creating solid-state lasers emitting at 488nm (the wavelength of argon lasers). The first commercial laser based on this technology, the Sapphire, was launched in 2001. Versions available today can provide two different colors in the 10–500mW power range.2
Table 1. Frequency-doubled wavelengths of different optically pumped semiconductor laser designs and maximum power achieved.
The OPS architecture lends itself to optimal scaling, achieved simply by increasing the beam size and pumped area on the OPS chip. Its wavelength flexibility and power-scaling capability are illustrated in Figure 2, which shows prototype lasers emitting multi-Watts of visible power at several different wavelengths.3 Among all the III-V semiconductor materials available, we selected frequency-doubled configurations based on chips grown in the InGaAs material system, yielding the operational wavelengths listed in Table 1. All chips designs display similar performance, and their different powers reflect the exploration of different laser architectures. In contrast to conventional solid-state lasers, the thermal lensing effect is minor for OPS-based systems,4 which opens an additional avenue for output power-scaling. Instead of further increasing the pump power to a chip, we can use multiple chips in the same laser cavity. For instance, the power listed in Table 1 for green was obtained with a three-chip laser aligned for slightly multimode operation.
Figure 2. Row of prototype optically pumped semiconductor lasers that emit different colors. All lasers share the same basic design and can emit multiple Watts of visible laser light.
Another advantage is that the system architecture required to achieve certain beam characteristics is much simpler than what would be required using conventional laser crystals. This includes not only the cavity configurations, but also the pump sources and control electronics, thus expanding the potential for new laser applications. We have recently released our first product featuring high-power OPS technology:5 the TracER, a portable, battery-operated laser system for field use in forensic applications. Another ongoing development is a laser for ophthalmic applications. Studies conducted with dye lasers showed that the preferred radiation for treatment was yellow, 577nm-light. Physicians today are compromising in favor of convenience by using green solid state lasers. With OPS, we can simply dial in the design to produce the wavelength most suitable for the application.
Semiconductors are in many ways ideal laser materials. By using them in conventional laser cavities instead of traditional laser crystals, we are able to fully exploit their properties and produce lasers that are best tailored to the intended application.
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